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.md

@@ -1,3 +1,16 @@
 # Integration
 
-TBD (pending Postgres support)
+Taskchampion-sync-server can be integrated into larger applications, such as
+web-based hosting services.
+
+- Most deployments can simply use the pre-built Docker images to implement the
+sync protocol, handling other aspects of the application in separate
+containers. See [Pre-built Images](./integration/pre-built.md).
+
+- More complex deployments may wish to modify or extend the operation of the
+server. These can use the Rust crates to build precisely the desired
+functionality. See [Rust Crates](./integration/crates.md).
+
+- If desired, an integration may completely re-implement the [sync
+protocol](https://gothenburgbitfactory.org/taskchampion/sync.html). See [Sync
+Protocol Implementation](./integration/protocol-impl.md).