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/crates.md

@@ -0,0 +1,17 @@
+# Rust Crates
+
+This project publishes several Rust crates on `crates.io`:
+
+- [`taskchampion-sync-server-core`](https://docs.rs/taskchampion-sync-server-core)
+implements the core of the protocol
+- [`taskchampion-sync-server-storage-sqlite`](https://docs.rs/taskchampion-sync-server-storage-sqlite)
+implements an SQLite backend for the core
+- [`taskchampion-sync-server-storage-postgres`](https://docs.rs/taskchampion-sync-server-storage-postgres)
+implements a Postgres backend for the core
+
+If you are building an integration with, for example, a custom storage system,
+it may be helpful to use the `core` crate and provide a custom implementation
+of its `Storage` trait.
+
+We suggest that any generally useful extensions, such as additional storage
+backends, be published as open-source packages.