The three shipped compose files forwarded the variable bare, so every
`docker compose` command on a default .env printed "variable is not set".
Default it to the 500 MiB that .env.example documents, like the other
limits, and add a guard test that rejects any bare ${VAR} in compose files.
Fixes#6619
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Alexandre Teixeira <general@alexandreteixeira.dev>
