This includes:
- Building a Docker image for Postgres as well as SQLite
- Fuller instructions for usage of the package, including the Postgres builds.
A few related things changed here:
- `.env` is not used anymore -- the defaults in the Dockerfiles are
sufficient
- The Rust version in the Dockerfiles is increased to match the MSRV,
and with it the Alpine version bumped to one built with that Rust
version.
- Cargo dependencies on native-tls and openssl updated to include only
the `vendored` feature, so as not to require a system openssl
installation.
- Two GitHub jobs are set up, to build the two different Docker images
- The documentation incorrectly suggested using `DELETE .. CASCADE` to
delete clients. This syntax does not exist, as the cascading delete
is configured in the schema.
