Add an example with PostgreSQL and Docker Compose.

[jsthomas]

This change addresses #99. It introduces an example illustrating how to use Docker Compose to run a Dream container and a PostgreSQL container. The example app server is essentially the same as an earlier example with small changes to use PostgreSQL instead of SQLite.

[aantron]

@jsthomas, looks great!

To get the CI to pass, could you, before this line

https://github.com/aantron/dream/blob/dd2b8dcdf458d0b95f91e0ae058bdfd71e28672a/.github/workflows/test.yml#L31

add - run: opam depext --yes conf-postgresql

and before this line

https://github.com/aantron/dream/blob/dd2b8dcdf458d0b95f91e0ae058bdfd71e28672a/dream.opam#L99

add "caqti-driver-postgresql" {with-test}

[jsthomas]

Ok, I believe CI should be all set now. Thanks for helping me fix that.

[aantron]

Thanks for the PR and the fix!