Setup Container Security

Harden a project's container image against supply-chain attack: pin the base, verify what you fetch, drop privileges, and scan what you ship. OCI-compliant first — target the OCI image spec so the artifact runs on any conformant runtime (podman, Docker, containerd). Build and test distributable images with podman (rootless, daemonless, OCI output by default), and stay Docker-compatible (Dockerfile/Containerfile are interchangeable below).

When to Use

• When a project builds a container image (has a Containerfile/Dockerfile)
• Before publishing an image to a registry (a project that ships the image as a release artifact)
• When auditing container security, or when a supply-chain review flags container gaps

Required Inputs

1. The project's Containerfile/Dockerfile(s)
2. Base image(s) and target registry
3. Whether the project publishes the image (ships it as a release artifact) — gates SBOM-as-release-asset + signing, delegated to wrapup-sprint

The Threat

A container bakes in a base image, fetched packages/binaries, and a runtime user. A mutable base tag can be repointed upstream; an unverified curl … | install can be swapped; a root runtime turns any escape into host-level access. Hardening: pin, verify, minimize, drop privileges, scan.

Workflow

Step 1: Triage

ls Containerfile Dockerfile **/Dockerfile 2>/dev/null
# Mutable base tags (should be digest-pinned):
grep -nE '^FROM .*:[^@]*$' Containerfile Dockerfile 2>/dev/null
# Runs as root? (no USER, or USER root):
grep -nE '^USER ' Containerfile Dockerfile 2>/dev/null || echo "no USER — root unless the entrypoint drops privileges"
# Unverified fetch-and-install:
grep -nE 'curl.*\|\s*(sh|bash)|wget .*http' Containerfile Dockerfile 2>/dev/null

The recurring "looks fine at a glance" anti-pattern (a real-world example): a -slim base on a mutable tag, no USER (root runtime), and a build-time binary/model download with no checksum — despite otherwise-good habits (--no-install-recommends, a HEALTHCHECK, a .containerignore). It fails the three load-bearing checks: digest pinning, non-root, and artifact integrity.

Step 2: Pin the base image by digest

Pin every FROM to the base's multi-arch OCI index digest (keep the tag for readability; preserves arm64/amd64):

# To bump: skopeo inspect --raw docker://docker.io/apache/age:<tag> | sha256sum
FROM apache/age:release_PG18_1.7.0@sha256:e7de1717…baf00

For a rolling minimal base (e.g. Chainguard), the :latest@sha256:… form lets Dependabot re-pin the digest while tracking the tag:

FROM cgr.dev/chainguard/node:latest@sha256:5280e6…22b3f AS runtime

Confirm you pinned the index, not a per-arch manifest:

podman image inspect <tag> --format '{{range .RepoDigests}}{{println .}}{{end}}'
skopeo inspect --raw docker://…@<digest>   # mediaType must be application/vnd.oci.image.index.v1+json

Step 3: Minimal base, multi-stage build

Keep the build toolchain out of the shipped image. Either a two-stage split with a distroless/Chainguard runtime:

FROM node:22-slim@sha256:… AS build
RUN npm ci --omit=dev --omit=optional --no-audit --no-fund && npm cache clean --force

FROM cgr.dev/chainguard/node:latest@sha256:… AS runtime
COPY --from=build --chown=65532:65532 /app /app

…or, when you must patch libs in place (distroless has no apt), a single stage that purges the toolchain in the same RUN layer, always with --no-install-recommends and apt-cache cleanup:

RUN set -eux; apt-get update; \
    apt-get install -y --no-install-recommends build-essential …; \
    make && make install; \
    apt-get purge -y --auto-remove build-essential …; \
    rm -rf /var/lib/apt/lists/*

Step 4: Run as non-root

Drop privileges with a USER before CMD. Distroless/Chainguard ships a nonroot uid (65532) you can switch to directly:

COPY --from=build --chown=65532:65532 /data /data
USER 65532

Context matters: some official bases (e.g. postgres) legitimately start as root and gosu-drop to a service user via their entrypoint — a "runs as root" finding there needs context, not a blind USER that breaks the entrypoint. Document that exception in an ADR rather than forcing a fix.

Step 5: Verify fetched artifacts

Anything downloaded at build time is attack surface — pin and verify:

• Distro/binary .deb — sha256-verify per arch before install:

  RUN set -eux; arch="$(dpkg --print-architecture)"; \
      case "$arch" in \
        arm64) sha256=59dbe75a… ;; amd64) sha256=7637a18a… ;; \
        *) echo "no pinned checksum for $arch" >&2; exit 1 ;; esac; \
      curl -fsSL -o /tmp/x.deb "https://…/${VERSION}/…_${arch}.deb"; \
      echo "${sha256}  /tmp/x.deb" | sha256sum -c -; \
      apt-get install -y --no-install-recommends /tmp/x.deb

• Source builds — clone a pinned git tag, not a branch: git clone --branch "${VERSION}" --depth 1 …, VERSION an ARG.
• Language deps — install from a committed lockfile (npm ci, mix from mix.lock) — see setup-git-hooks.

Never curl … | sh an installer, and never fetch a binary/model with no checksum gate.

Step 6: .dockerignore

Keep .git, secrets, and tests out of the build context (Podman honours .containerignore too):

.git
.github
.env
.env.*
CLAUDE.local.md
test

Step 7: Lint — hadolint + shellcheck

Lint the Dockerfile with hadolint and build scripts with shellcheck, in both the pre-push hook and CI:

# .lefthook.yml (pre-push)
lint-dockerfile:
  glob: "Dockerfile*"
  run: hadolint {push_files}
lint-shell:
  glob: "*.sh"
  run: shellcheck {push_files}

Prefer inline, justified waivers over a blanket .hadolint.yaml — they document why each rule is suppressed, at the line it applies to:

# hadolint ignore=DL3008,DL4006

Step 8: Scan — grype + syft SBOM

Scan the built image for known CVEs and generate an SBOM. Export the image to an archive so both run daemonless:

podman save --format oci-archive -o img.tar "$IMAGE"      # src="oci-archive:img.tar"
grype "oci-archive:img.tar"                                # reads .grype.yaml from repo root
syft scan "oci-archive:img.tar" -q \
  -o spdx-json=sbom.spdx.json -o cyclonedx-json=sbom.cdx.json

Commit a .grype.yaml policy that fails only on fixable High/Critical — a defensible anti-alert-fatigue gate that auto-resurfaces a finding when a fix ships:

fail-on-severity: "high"
ignore:
  - fix-state: wont-fix
  - fix-state: not-fixed
  - fix-state: unknown

Run the scan in CI on image build/PR, plus a weekly cron re-scan of the pinned image as a drift backstop.

Step 9: HEALTHCHECK

Add a HEALTHCHECK. In a distroless image (no shell), probe with the runtime instead of sh:

HEALTHCHECK --interval=10s --timeout=3s --retries=6 \
  CMD ["/usr/bin/node","-e","require('net').connect(PORT,'127.0.0.1').on('connect',c=>{c.end();process.exit(0)}).on('error',()=>process.exit(1))"]

Note: Podman's default OCI image format drops HEALTHCHECK — carry a duplicate in compose.yaml for Compose users.

Step 10: Automate digest bumps

Add a docker ecosystem to .github/dependabot.yml so base-image digests get weekly bump PRs (see bootstrap-project), each validated by the grype scan job.

Step 11: Publishing and ADR

For projects that publish the image: attach the SBOM as a release asset and sign the image with cosign keyless — delegated to wrapup-sprint's release-provenance step. Record the container-security decisions (which checks run, the grype-policy rationale, and any documented exceptions like a base-digest-pin or gosu-drop root) in an ADR.

Grounding: EU CRA (SBOM, integrity) and OpenSSF supply-chain hardening. The base-digest-pin + .deb sha256 + git-tag builds close the "install whatever arrives over the network" gap; the fixable-only grype policy keeps the gate actionable.

Outputs

• Every FROM digest-pinned (multi-arch index), with a bump-command comment
• Multi-stage or same-layer-purge build; --no-install-recommends + apt cleanup
• USER drops privileges before CMD (or the root exception is ADR-documented)
• Fetched .deb/binaries sha256-verified; source builds git-tag-pinned; no curl | sh
• .dockerignore / .containerignore excludes .git, secrets, tests
• hadolint + shellcheck in pre-push + CI (inline justified ignores)
• .grype.yaml (fixable-only policy) + grype scan in CI + weekly re-scan
• syft SBOM (SPDX + CycloneDX)
• HEALTHCHECK
• docker Dependabot ecosystem for digest bumps
• ADR recording the container-security checks and documented exceptions

Validation

# No mutable base tags:
grep -nE '^FROM .*:[^@]*$' Dockerfile Containerfile 2>/dev/null && echo "UNPINNED base above" || echo "bases pinned"
# Non-root (or documented exception):
grep -qE '^USER ' Dockerfile Containerfile || echo "no USER — confirm the entrypoint drops privileges"
# Scan passes under policy + SBOM produced:
grype "oci-archive:img.tar"
test -s sbom.spdx.json && test -s sbom.cdx.json && echo "SBOM ok"

Related Skills

• harden-github-actions - harden the image build/publish workflow (SHA-pin actions, least-privilege, cosign-installer)
• wrapup-sprint - release-time SBOM + cosign signing for published images
• setup-git-hooks - lockfile enforcement and the hadolint/shellcheck hooks
• bootstrap-project - the distribution profile (whether the project publishes artifacts); Dependabot config
• setup-adrs - record the container-security checks and documented exceptions