Containerfile Creator

Containerfile Creator

Create secure, efficient, and maintainable container images following open source best practices.

Workflow

1. Determine the project language/framework
2. Check host environment for SELinux enforcement (e.g. getenforce) if using Podman/Buildah
3. Load the appropriate reference file for examples (see Language-Specific References below)
4. Apply the core structure and patterns from this document
5. Write the Containerfile following the Final Stage Instruction Ordering

File Naming

• Prefer Containerfile over Dockerfile
• Use descriptive names for variants: alpine.Containerfile, distroless.Containerfile, ubi.Containerfile

Required Syntax Declaration

Every file MUST start with:

# syntax=docker/dockerfile:1

ARG Definition Block

Only UID, VERSION, and RELEASE go at the top level:

# syntax=docker/dockerfile:1
ARG UID=1001
ARG VERSION=EDGE
ARG RELEASE=0

TARGETARCH and TARGETVARIANT MUST be declared inside each stage that uses them, not globally.

Multi-stage Build Structure

########################################
# Build stage
########################################
FROM python:3.13-alpine AS build

########################################
# Final stage
########################################
FROM python:3.13-alpine AS final

• Always name the last stage final
• Use 40 # characters for stage separators
• Only use a separate base stage when it requires non-trivial setup (e.g., installing system packages on UBI/Debian)

Cache Optimization

Declare multi-arch variables inside each stage that uses cache mounts:

ARG TARGETARCH
ARG TARGETVARIANT

Package Manager Cache Patterns

# Alpine APK
RUN --mount=type=cache,id=apk-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/var/cache/apk \
    apk update && apk add -u package-name

# Debian/Ubuntu APT
RUN --mount=type=cache,id=apt-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/var/cache/apt \
    --mount=type=cache,id=aptlists-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/var/lib/apt/lists \
    apt-get update && apt-get install -y --no-install-recommends package-name

# Python PIP
RUN --mount=type=cache,id=pip-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/root/.cache/pip \
    pip install package-name

# Python UV
RUN --mount=type=cache,id=uv-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/root/.cache/uv \
    uv pip install package-name

# DNF (Fedora/RHEL)
RUN --mount=type=cache,id=dnf-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/var/cache/dnf \
    dnf -y install package-name

Security and Permissions

Create Non-root User

# Alpine
ARG UID
RUN adduser -g "" -D $UID -u $UID -G root

# Debian/Ubuntu
ARG UID
RUN groupadd -g $UID $UID && \
    useradd -l -u $UID -g $UID -m -s /bin/sh -N $UID

OpenShift Compatibility (Arbitrary UID)

# Create directories
RUN install -d -m 775 -o $UID -g 0 /app && \
    install -d -m 775 -o $UID -g 0 /licenses

# Copy files with proper ownership
COPY --link --chown=$UID:0 --chmod=775 source dest

License Files (Required)

COPY --link --chown=$UID:0 --chmod=775 LICENSE /licenses/Containerfile.LICENSE
COPY --link --chown=$UID:0 --chmod=775 project/LICENSE /licenses/project.LICENSE

SELinux Compatibility (Rootless Podman / Bind Mounts)

By default, all code examples in this skill assume standard environments without SELinux or default Docker daemon execution. However, on SELinux-enforcing systems (such as Fedora, RHEL, CentOS) when using Podman or Buildah, --mount=type=bind instructions will fail with Permission denied (os error 13) because the container process cannot access host files without explicit SELinux security context relabeling.

SELinux Workflow & Verification Guidelines

1. Verify System Status: Check if SELinux is active (e.g. getenforce outputs Enforcing).
2. Inform / Consult User:
   • Inform the user if an SELinux environment is detected and explain why ,relabel=shared (or ,relabel=private) is required on --mount=type=bind.
   • Explicitly confirm with the user or state whether SELinux-compatible syntax (relabel=shared) or standard Docker syntax is being used.
3. Containerfile & Documentation Requirements:
   • When SELinux relabeling is adopted, clearly document it with inline comments in the Containerfile / Dockerfile:

     # Note: relabel=shared is required for SELinux (Podman/Fedora/RHEL) rootless bind mounts
     RUN --mount=type=cache,id=uv-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/root/.cache/uv \
         --mount=type=bind,source=pyproject.toml,target=pyproject.toml,relabel=shared \
         --mount=type=bind,source=uv.lock,target=uv.lock,relabel=shared \
         uv sync --frozen --no-dev --no-install-project --no-editable
     

   • Document the SELinux build requirements in README.md or project documentation whenever podman build or podman compose build is a supported build workflow.

Secure dumb-init Usage

Use dumb-init as PID 1 for proper signal handling. Download in a separate stage with SHA256 verification:

########################################
# Download stage
########################################
FROM docker.io/library/debian:bookworm-slim AS download

ARG TARGETARCH
ARG TARGETVARIANT

RUN --mount=type=cache,id=apt-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/var/cache/apt \
    --mount=type=cache,id=aptlists-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/var/lib/apt/lists \
    apt-get update && apt-get install -y --no-install-recommends curl ca-certificates

# Download dumb-init static binary (arch-aware) with SHA256 verification
RUN case "${TARGETARCH}" in \
      amd64) DUMBINIT_ARCH="x86_64"; DUMBINIT_SHA256="e874b55f3279ca41415d290c512a7ba9d08f98041b28ae7c2acb19a545f1c4df" ;; \
      arm64) DUMBINIT_ARCH="aarch64"; DUMBINIT_SHA256="b7d648f97154a99c539b63c55979cd29f005f88430fb383007fe3458340b795e" ;; \
      *) echo "unsupported architecture: ${TARGETARCH}" && exit 1 ;; \
    esac && \
    curl -fsSL "https://github.com/Yelp/dumb-init/releases/download/v1.2.5/dumb-init_1.2.5_${DUMBINIT_ARCH}" \
    -o /dumb-init && \
    echo "${DUMBINIT_SHA256}  /dumb-init" | sha256sum -c -

Then copy in the final stage:

COPY --link --chown=$UID:0 --chmod=775 --from=download /dumb-init /usr/local/bin/dumb-init

OpenShift umask Pattern

When the application creates directories at runtime, use umask 0002 to preserve GID-0 group-write:

ENTRYPOINT ["dumb-init", "--"]
CMD ["sh", "-c", "umask 0002 && exec my-app"]

The exec replaces the shell so dumb-init's signal forwarding reaches the app directly.

COPY --link Optimization

Always use --link flag for COPY instructions to enable layer reuse across builds:

COPY --link --chown=$UID:0 --chmod=775 source dest

Do NOT use --link when destination path contains symlinks that need to be followed.

Final Stage Instruction Ordering (CRITICAL)

Follow this exact order in the final stage:

1. System cleanup (remove pip/setuptools/wheel if applicable)
2. Create user (non-root)
3. Create directories (install -d)
4. COPY from build (--link --chown=$UID:0 --chmod=775)
5. ENV (PATH and other variables)
6. WORKDIR
7. VOLUME (if applicable)
8. EXPOSE (if applicable)
9. USER $UID
10. STOPSIGNAL SIGINT
11. ENTRYPOINT / CMD
12. ARG VERSION + ARG RELEASE + LABEL — ALWAYS LAST

LABEL MUST be the very last instruction. VERSION/RELEASE ARGs bust the cache for all subsequent instructions. Placing them last ensures maximum cache reuse.

LABEL Standards

ARG VERSION
ARG RELEASE
LABEL name="project-name" \
    vendor="original-author" \
    maintainer="user-id" \
    url="https://github.com/user-id/project" \
    version=${VERSION} \
    release=${RELEASE} \
    io.k8s.display-name="Display Name" \
    summary="Brief summary" \
    description="Detailed description with website reference"

Health Check

HEALTHCHECK does not function in OCI/podman builds. Do NOT implement unless specifically requested.

When implementing:

COPY --link --from=ghcr.io/tarampampam/curl:8.7.1 /bin/curl /usr/local/bin/
HEALTHCHECK --interval=30s --timeout=2s --start-period=30s \
    CMD [ "curl", "--fail", "http://localhost:8080/" ]

Test, Report, and Binary Stages (Optional)

Test Stage

Run linting, type-checking, and tests inside the build. Use --mount=type=bind for test files to avoid caching them in layers:

########################################
# Test stage
########################################
FROM deps AS test

ARG TARGETARCH
ARG TARGETVARIANT

ENV PATH="/venv/bin${PATH:+:${PATH}}"

WORKDIR /app

# Install dev dependencies using separate cache to avoid conflicts
RUN --mount=type=cache,id=uv-test-$TARGETARCH$TARGETVARIANT,sharing=locked,target=/root/.cache/uv \
    --mount=type=bind,source=pyproject.toml,target=/app/pyproject.toml \
    --mount=type=bind,source=uv.lock,target=/app/uv.lock \
    uv sync --frozen --no-install-project

COPY src/ src/

# Run quality checks and tests with bind-mounted test files
RUN --mount=type=bind,source=tests,target=/app/tests \
    --mount=type=bind,source=pyproject.toml,target=/app/pyproject.toml \
    pytest --junit-xml=/app/test-results.xml \
           --cov=src \
           --cov-report=xml:/app/coverage.xml \
           --cov-fail-under=70 \
           --verbose

Report Stage

Extract test reports from the build without running the full image:

########################################
# Report stage
# How to: podman build --target report --output type=local,dest=./out .
########################################
FROM scratch AS report

ARG UID=1001
COPY --chown=$UID:0 --chmod=775 --from=test /app/test-results.xml /
COPY --chown=$UID:0 --chmod=775 --from=test /app/coverage.xml /

Extract reports locally:

podman build --target report --output type=local,dest=./out .

Binary Stage

Export statically linked binaries from the build:

########################################
# Binary stage
# How to: podman build --output=. --target=binary .
########################################
FROM scratch AS binary
COPY --from=builder /app/binary /

Binary stage is only for statically linked binaries or self-contained outputs.

Layer Cache Strategy

Install large, rarely changed packages first, then project-specific dependencies:

RUN uv pip install torch==2.7.0 tensorflow>=2.16.1
RUN uv pip install -r requirements.txt

.containerignore/.dockerignore Template

**/node_modules
**/*.log
**/.git
**/.gitignore
**/.env
**/.github
**/.vscode
**/bin
**/obj
**/dist
**/tmp

Language-Specific References

Load the appropriate reference file based on the project's language/pattern:

Python

• python--package-manager-patterns.md — UV/pip config snippets, environment setup
• python--pip-cli-app.md — pip-based CLI app + base image variants
• python--nuitka-standalone.md — Nuitka binary compilation
• python--ml-gpu-cuda.md — ML/CUDA with GPU support
• python--grpc-service-with-test.md — Web service with test/report/codegen stages

Other Languages

• golang--upx-busybox-ubi.md — Go with UPX compression, busybox/UBI targets
• rust--cargo-chef-static.md — cargo-chef, static linking, binary export
• nodejs--alpine.md — npm ci, Alpine
• deno--vue-frontend.md — Deno cache, Vue frontend, secure dumb-init

Infrastructure / Patterns

• dotnet--self-contained.md — .NET self-contained deployment
• scratch--static-binary.md — Minimal scratch image with ADD
• nginx--frontend-spa.md — Frontend build → nginx serve
• alpine--single-stage-apk.md — Single-stage Alpine with apk
• fedora-toolbox--devenv.md — Development environment container