Finish building Postgres support (#133)

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.

postgres/Cargo.toml

@@ -21,8 +21,9 @@ thiserror.workspace = true
 tokio-postgres.workspace = true
 tokio.workspace = true
 uuid.workspace = true
-native-tls = { version = "0.2.14", features = ["vendored"] }
-postgres-native-tls = "0.5.1"
+openssl.workspace = true
+native-tls.workspace = true
+postgres-native-tls.workspace = true
 
 [dev-dependencies]
 tempfile.workspace = true