Compose: init containers

[aevesdocker]

Description

As per docker/compose#13862 and compose-spec/compose-spec#647

Related issues or tickets

Reviews

• Technical review
• Editorial review
• Product review

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[docker-agent]

Assessment: 🔴 CRITICAL

This PR adds documentation for the new pre_start lifecycle hook. Three issues need attention before merging: one unresolved internal placeholder that would ship to users, and two factual inconsistencies between the how-to guide and the reference page.

[glours]

Just 1 little nit to fix before merging, thanks @aevesdocker 🙏

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

Two medium-severity issues found in the new pre_start reference documentation added by this PR. Both are in content/reference/compose-file/services.md.

The new how-to page (content/manuals/compose/how-tos/init-containers.md) is well-written and clear. No missing redirects, no vendored content concerns, no front matter issues.

[usha-mandya]

LGTM

[dvdksn]

Three issues with the Postgres examples that need fixing before merge.

[dvdksn]

This example is confusing because it mixes two unrelated concerns in one pre_start sequence:

1. ./manage.py migrate — a DB migration (needs the db service)
2. chown -R 1000:1000 /data — fixing ownership of a volume

The app service has volumes: data:/data but the db service has no volume. A reader trying to understand pre_start from this example will wonder why the app is chowning a path that the database doesn't share, and why the database has no persistent volume of its own.

Also, db: image: postgres:16 has no POSTGRES_PASSWORD, POSTGRES_USER, or POSTGRES_DB — the postgres container won't start.

Suggest splitting into two focused examples: one for migrations (the DB case, already done in init-containers.md) and one for volume ownership (the busybox/chown case, already done separately in init-containers.md). The combined example here adds confusion rather than showing a realistic use case.

[dvdksn]

Two problems with this db service that would prevent it from actually working:

1. No POSTGRES_PASSWORD — Postgres refuses to start without it (or POSTGRES_HOST_AUTH_METHOD=trust), with the error: Error: Database is uninitialized and superuser password is not specified. You must specify POSTGRES_PASSWORD to a non-empty value for the superuser.

2. Version inconsistency — this example uses postgres:18 but the services.md reference example uses postgres:16. Pick one version and use it consistently across both files.

At minimum the db service should look like:

db:
image: postgres:17environment:
POSTGRES_PASSWORD: examplePOSTGRES_USER: myuserPOSTGRES_DB: mydbhealthcheck:
test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]interval: 10sretries: 5start_period: 30stimeout: 10s

[dvdksn]

Same issue here: postgres:18 with no POSTGRES_PASSWORD — this db container won't start. Same version inconsistency vs services.md (postgres:16).