8aa400a5d4
Add 37 new entries and update 7 existing entries across 13 topic files. Major contributions from agent-runtimes (K8s secrets, CI, Docker gotchas), cluster-bootstrap (ArgoCD SSA, etcd tuning, DB migrations, Compose networking), and cluster-apps/octopus-deploy (Helm vs raw manifests, ArgoCD source types). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
76 lines
6.6 KiB
Markdown
76 lines
6.6 KiB
Markdown
# Docker Best Practices
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## Use gosu for Entrypoint Privilege Dropping
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`su -c "command"` and `sudo -u agent command` create child processes. The real command is not PID 1, so Docker signals (SIGTERM on stop) don't reach it. Use `gosu agent command` which execs directly — the command becomes PID 1 with proper signal handling.
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## GIT_SSH_COMMAND Only Affects Git-Invoked SSH
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`GIT_SSH_COMMAND` only applies when git invokes SSH (clone, push, fetch). Direct `ssh` calls need explicit flags: `ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null`. Don't assume setting `GIT_SSH_COMMAND` fixes all SSH operations in a container.
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## Use Python urllib for Health Checks in Slim Images
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Service images based on `python:3.12-slim` don't include curl. For in-container health checks, use `python3 -c "import urllib.request; urllib.request.urlopen('http://...')"`. This applies to verification scripts using `kubectl exec` and to Kubernetes liveness/readiness probes that exec into containers.
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## Buildx Docker-Container Driver Can't See Local Images
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When using buildx with the docker-container driver, `FROM local-image:latest` tries Docker Hub because the builder runs in a separate container that can't see locally-loaded images. Always use the full registry path in Dockerfiles. In CI, split into sequential jobs so base images are pushed to the registry before dependent images build.
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## Delete Conflicting Default Users at Build Time
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Ubuntu 24.04 base images ship with a `ubuntu` user at UID 1000 — the most common host UID. This causes `usermod -u 1000` conflicts and can trigger non-deterministic hangs (e.g., `newgrp ubuntu` waiting for a password on stdin). Delete the default user in the Dockerfile: `RUN userdel -r ubuntu`.
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## Service Images Should Use Minimal Base Images
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Service images (API servers, background workers) should use `python:3.12-slim` or equivalent, not the agent base image. Agent base images include CLIs, Node.js, and other tooling that bloats service images unnecessarily. Keep agent tooling in agent images only.
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## Platform-Specific Native Binaries
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Never mount host `node_modules` into a Docker container when the build uses platform-specific native binaries (e.g., Tailwind CSS, esbuild, SWC). Always run `npm install` inside the same container that runs the build. The native binary is compiled for the platform where `npm install` runs — host and container may differ in libc, architecture, or OS.
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**Symptom:** `Cannot find native binding` or `Cannot find module '@tailwindcss/oxide-linux-x64-gnu'`
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**Fix:** Run `npm install` inside the container, not on the host.
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## Docker Wrapper Scripts and TTY Flags
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Docker wrapper scripts (e.g., `~/sbin/hugo` calling `docker run -it ...`) fail with `the input device is not a TTY` in non-interactive contexts (CI pipelines, Claude Code, cron jobs, scripts).
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**Fix:** Only pass `-t` when stdin is a terminal: `[ -t 0 ] && TTY_FLAG="-t" || TTY_FLAG=""`. Or omit `-t` entirely and let callers add it when needed.
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## Three-Tier UID Resolution
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The UID wrapper should resolve the target UID/GID using this priority:
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1. **Environment variables** (`AGENT_UID`/`AGENT_GID`) — injected by the orchestrator/dispatcher. Preferred because it's explicit and deployment-specific.
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2. **stat the mount point** — detect the UID/GID of the mounted directory. Works when no env vars are set.
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3. **Skip** — if not root or no mount point exists, run as the default container user.
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This makes UID matching a deployment concern (varies per host), not a configuration concern (baked into images). See [Docker UID Matching](docker-uid-matching.md) for the full UID wrapper pattern.
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## network_mode: service:* Breaks on Parent Container Restart
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When a container shares another's network namespace via `network_mode: "service:<parent>"`, restarting the parent recreates the namespace. The dependent container keeps stale socket bindings — its listeners are bound to a namespace that no longer exists. TCP connections fail while the dependent container appears healthy. Fix: add a healthcheck to the parent and use `depends_on: condition: service_healthy` on the dependent. Alternatively, use an internal bridge network instead of namespace sharing.
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## docker compose restart Is Concurrent, Not Ordered
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`docker compose restart svc1 svc2 svc3` restarts all named services concurrently, ignoring `depends_on` ordering. Dependent services may start before their dependencies are ready. Use `docker compose up -d` (which respects `depends_on`) or restart in explicit stages: stop dependents, restart the dependency, wait for healthy, then start dependents.
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## docker cp Can Corrupt Container Filesystem Ownership
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`docker cp` runs as root and can change ownership of parent directories in the container's filesystem layer. This is particularly dangerous for database containers (e.g., PostgreSQL UID 999) — copying a file into `/tmp/` can corrupt the data directory ownership, causing "Permission denied" errors. After any `docker cp` into a stateful container, verify and fix ownership: `chown -R <expected-uid>:<expected-gid> <data-dir>`.
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## Prefer Internal Bridge Networks Over Namespace Sharing
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For sidecar-style containers that need to communicate (e.g., app + database, app + TLS proxy), prefer an internal Docker bridge network over `network_mode: "service:<parent>"`. Bridge networks allow proper `depends_on` ordering with health checks, independent restart of each container, and clear network isolation. Namespace sharing couples container lifecycles — restarting the parent invalidates the dependent's network stack.
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## Verify Dockerfile COPY After Creating New Files
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After creating a file intended for a Docker image (scripts, configs, wrappers), immediately add the COPY line to the Dockerfile and verify with `docker run --entrypoint sh <image> -c "ls /path/to/file"`. Files placed in image source directories are not automatically included — they need explicit COPY instructions. This class of bug can remain latent until the code path is first exercised.
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## docker-compose.override.yaml Merges Lists Additively
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Docker Compose V2 merges list fields (ports, volumes, environment) by appending, not replacing. An override file with a different port mapping adds a second binding rather than replacing the original, causing conflicts. Modify the base `docker-compose.yaml` directly or use `!override` for list replacement.
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## Volume Source Paths Must Be Absolute
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Docker interprets relative paths in volume mount source fields as named volumes, not bind mounts. Use `os.path.abspath()` or equivalent when constructing volume source paths programmatically. The error message ("includes invalid characters for a local volume name") is misleading — the real issue is that the path is relative.
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