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/src/lib.rs

@@ -25,7 +25,7 @@
 //!    `taskchampion-sync-server` to never call this method.
 //!  - Insert rows into the `clients` table, using default values for all columns except
 //!    `client_id` and application-specific columns.
-//!  - Delete rows from the `clients` table, using `CASCADE` to ensure any associated data
+//!  - Delete rows from the `clients` table, noting that any associated task data
 //!    is also deleted.
 
 use anyhow::Context;