fix(docker): correct compose example after actual e2e test

[buremba]

Summary

Fixes two real bugs in the sample compose shipped in #850 that I should have caught before merging — applying the e2e-before-merge rule that #837 codified hours earlier. My bad.

Reproducer (the test #850 should have included)

cp docker-compose.example.yml /tmp/test/docker-compose.yml
cd /tmp/test
# Generate the two secrets per docs/DOCKER.md
sed -i '' "s|REPLACE_ME_openssl_rand_base64_32|$(openssl rand -base64 32)|"  docker-compose.yml  # ENCRYPTION_KEY
sed -i '' "0,/REPLACE_ME_openssl_rand_base64_32/{s/REPLACE_ME_openssl_rand_base64_32/$(openssl rand -base64 32)/}" docker-compose.yml  # BETTER_AUTH_SECRET
docker compose up -d

Before this fix

Container lobu-postgres Error dependency postgres failed to start
dependency failed to start: container lobu-postgres is unhealthy

Postgres logs (truncated):

Counter to that, there appears to be PostgreSQL data in:
  /var/lib/postgresql/data (unused mount/volume)
This is usually the result of upgrading the Docker image without
upgrading the underlying database...

And on Apple Silicon, even before postgres tries to start:

no matching manifest for linux/arm64/v8 in the manifest list entries

After this fix

Container lobu-postgres Healthy
Container lobu-app Started

Server logs end with:

{"port":8787,"msg":"Starting server"}
{"host":"0.0.0.0","port":8787,"msg":"Server running at http://0.0.0.0:8787"}

GET /healthz → {"status":"ok",...}. GET / returns 3211 bytes of SPA HTML. All 16 bundled providers loaded, 22 migrations applied cleanly.

The two fixes

1. Volume mount: lobu_pgdata:/var/lib/postgresql/data → lobu_pgdata:/var/lib/postgresql. PG18+ uses major-version-specific subdirectories under the parent dir, so mounting /data directly conflicts with that layout and PG refuses to start. The user's original Docker deployment crashes with no error message — impossible to debug locally #766 compose had this right; I "fixed" it during docs: docker-compose example + self-hosting guide #850 and broke it.

2. Apple Silicon platform: linux/amd64: the published image is amd64-only. Added a commented-out platform: line users can uncomment, plus a note in docs/DOCKER.md. Multi-arch image build is a follow-up.

Test plan

• docker compose -f docker-compose.example.yml config validates after edits.
• Real e2e: compose up → both containers healthy → GET /healthz 200 → GET / returns SPA → no errors in lobu-app logs.

Refs #766.

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📒 Files selected for processing (2)

• docker-compose.example.yml
• docs/DOCKER.md

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• Create PR with unit tests
• Commit unit tests in branch fix/docker-compose-example

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