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.

docs/src/integration/protocol-impl.md

@@ -0,0 +1,10 @@
+# Sync Protocol Implementation
+
+The [sync protocol](https://gothenburgbitfactory.org/taskchampion/sync.html) is
+an open specification, and can be re-implemented from that specification as
+desired. This specification is not battle-tested, so refer to
+taskchampion-sync-server's implementation to resolve any ambiguities, and
+please create pull requests to resolve the ambiguity in the specification.
+
+We suggest that new implementations be published as open-source packages where
+possible.