Dockerfile layer, cache, and build-context hygiene

A tool-agnostic recipe to make container builds reproducible, cache-efficient, and free of accidental context. It supports a read-only audit and, when explicitly authorized, a narrow remediation.

When to use it

Use this recipe when the operator’s specific objective is to make container builds reproducible, cache-efficient, and free of accidental context. Prefer a narrower CVE, scanner-finding, security-audit, or compliance-evidence recipe when that is the operator’s actual job.

Inputs

  • Repository root and the files, package, service, or module in scope.
  • Requested mode: audit or explicitly authorized fix.
  • Supported runtime, compiler, framework, operating-system, and deployment versions inferred from repository files.
  • Existing formatter, compiler, analyzer, test, build, and package-manager commands.
  • Public API, compatibility, performance, generated-code, vendor, and migration constraints.

The prompt

You are running the Security Recipes code-hygiene workflow `code-hygiene.platform.dockerfile-layer-cache-and-build-context`.

Start read-only. Do not edit files until the operator explicitly authorizes a fix.
Use repository configuration and installed tool versions as authoritative; do not impose a new style or toolchain.

### Scope

Your bounded objective is to make container builds reproducible, cache-efficient, and free of accidental context.
Inspect only operator-scoped, first-party source and configuration. Exclude generated, vendored, minified, fixture snapshot, lock history, and migration history unless explicitly included.

### Detection

- Inspect Dockerfile stage graph, context, dockerignore, copy order, package-manager caches, build arguments, and emitted artifacts.
- Compare clean and cached builds and final-image contents.
- Record file and symbol evidence for every candidate. Mark uncertain or dynamically reachable behavior instead of guessing.

### Fix, only when authorized

- Narrow context and stages, order stable inputs before volatile inputs, and remove build-only material from final stages.
- Keep the diff limited to the proven issue and its focused tests. Preserve public behavior, API compatibility, and repository conventions.
- Do not add or broaden suppressions, weaken diagnostics, mass-format unrelated files, upgrade dependencies, or mutate external state.

### Verification

- Run clean and cached builds, inspect final files and history, and execute existing image tests.
- Compare diagnostic counts and relevant behavior before and after. Report every command, result, and check that could not run.

### Stop conditions

- Stop before changing base versions, package versions, registry, signing, or release provenance.
- Stop and hand off to a focused security recipe if evidence indicates a vulnerability, secret exposure, authorization failure, injection path, or named CVE.
- Stop rather than widening scope when the safe result requires architecture, product, compliance, operational, or data-owner decisions.

Output contract

  • Scope and repository evidence reviewed.
  • A candidate table with file or symbol, evidence, confidence, and disposition.
  • In audit mode: no edits, plus the smallest safe next action for each confirmed item.
  • In fix mode: one bounded patch, focused regression coverage, and no unrelated cleanup.
  • Commands run, results, remaining uncertainty, and any stop-condition handoff.

Verification

  • Run clean and cached builds, inspect final files and history, and execute existing image tests.
  • Confirm that diagnostic configuration, suppressions, public interfaces, generated files, vendored files, and dependencies did not change outside the authorized scope.
  • Review the final diff for behavior changes and run the repository’s focused checks before broader suites.

Guardrails

  • Read-only until edits are explicitly authorized.
  • Do not deploy, publish, rotate secrets, alter cloud or database state, change CI permissions, or open external tickets.
  • Do not hide debt by disabling rules, adding retries or sleeps, weakening tests, broadening ignores, or lowering warning levels.
  • Treat generated, vendored, minified, migration-history, and fixture-snapshot files as out of scope unless explicitly named.
  • Stop before changing base versions, package versions, registry, signing, or release provenance.

References