Introduction

If Docker seems to use an existing image and skip rebuilding with Docker Compose, that is usually expected behavior based on cache and Compose configuration. Compose decides whether to build or pull according to your docker-compose.yml, image tags, build directives, and CLI flags. Without explicit rebuild instructions, it may start containers from previously built images. Understanding this behavior prevents confusion when code changes are not reflected in running containers.

Core Sections

How Compose decides build vs image use

Common configurations:

• image: only -> pull/use image,
• build: only -> build from Dockerfile,
• both build: and image: -> build and tag or reuse depending on command/flags.

Example:

1services:
2  web:
3    build: .
4    image: myapp:web

docker compose up may reuse cached layers and existing image tags if no invalidating change exists.

Force rebuild when needed

Use explicit build flags:

docker compose build --no-cache
docker compose up --build

If stale containers persist, recreate:

docker compose up --build --force-recreate

Ensure changed files are in build context

If source changes are outside build context or excluded by .dockerignore, rebuild will not include them.

COPY . /app

Verify working directory and compose context paths.

Tag hygiene and pull behavior

Using mutable tags like latest can hide where an image came from. Prefer explicit version tags and controlled pulls.

docker compose pull
docker compose up -d

In CI, pin image digests for deterministic deployments.

Debug what container actually runs

Inspect image ID used by a service:

docker compose ps
docker inspect <container_id> --format '{{.Image}}'

Compare with locally built image IDs.

Common Pitfalls

• Expecting docker compose up to rebuild automatically without --build.
• Editing files outside the configured build context and seeing no image changes.
• Relying on mutable tags and losing traceability of deployed image versions.
• Confusing running container state with newly built image state.
• Ignoring .dockerignore rules that exclude needed files.

Verification Workflow

After build changes, run an explicit rebuild and verify image IDs before deployment. Add a startup log line with app version or commit hash so running containers are easy to confirm. In CI/CD, include build metadata labels and reject deployments where expected version markers are missing.

11. Build with explicit flags
22. Recreate containers
33. Inspect running image ID
44. Verify app version/commit output
55. Enforce tag or digest policy in CI

Production Readiness Checklist

Before considering the implementation complete, run a repeatable readiness pass that validates correctness, failure handling, and operational behavior in the same environment class where this solution will run. Start with a deterministic happy-path example and then exercise one malformed input and one resource-constrained scenario. Capture structured output such as status codes, key counters, and timing metrics so regressions are visible across revisions.

Document expected behavior boundaries in plain language so future maintainers can quickly understand what is guaranteed and what is best-effort. If configuration affects behavior, include the exact setting names and safe defaults in your runbook. For team workflows, add one lightweight automated check in CI to enforce these expectations on every change and keep debugging effort low when dependencies or runtime versions change.

11. Validate normal input path
22. Validate malformed or missing input path
33. Validate constrained-resource behavior
44. Record timing and error metrics
55. Confirm rollback or fallback behavior
66. Add CI smoke check for regression detection

Summary

Compose reusing an image is usually correct behavior, not a bug. Control rebuilds explicitly, keep build context accurate, and verify running image identity with inspection tools. With clear tagging and rebuild policies, container updates become predictable.