fix: pin postgres image to bookworm in docker_example

[erichare]

Summary

Pin the PostgreSQL image in docker_example/ from postgres:16 to postgres:16-bookworm to prevent a recurring collation version mismatch warning on existing langflow-postgres volumes.

Background

Users running Langflow with the example docker-compose.yml and pull_policy: always see this warning after Docker Hub silently rolled the postgres:16 tag from Debian Bookworm (glibc 2.36) to Debian Trixie (glibc 2.41):

WARNING: database "langflow" has a collation version mismatch
DETAIL: The database was created using collation version 2.36, but the
operating system provides version 2.41.

The warning is harmless in most cases but floods logs and creates user confusion. Reported in #9608.

Root cause

docker_example/docker-compose.yml used postgres:16 without a base-OS qualifier. When Docker Hub updated postgres:16 from a Bookworm-based build to a Trixie-based build, any user with pull_policy: always pulled the new image and started it against a data directory initialized under the older glibc collation version.

Fix

• Pin to postgres:16-bookworm in docker_example/docker-compose.yml and docker_example/pre.docker-compose.yml.
• Add inline comments explaining the pin and linking to issue database "langflow" has a collation version mismatc #9608.
• Update docker_example/README.md so the documented image tag matches.

Bookworm specifically (vs. Trixie) keeps the OS locale data stable for users with pre-existing volumes initialized under glibc 2.36, while still receiving 16.x patch updates.

Test plan

• docker compose -f docker_example/docker-compose.yml up starts cleanly
• No collation version mismatch warning on container startup
• Existing langflow-postgres volumes (initialized under glibc 2.36) start without the warning
• langflow service can still connect to the pinned postgres

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch fix/postgres-version

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[vjgit96]

we recently updated deb base image to trixie from bookworm(buid-time) so just curious to know why not trixie and just bookworm?

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