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/usage/binaries.md

@@ -0,0 +1,49 @@
+# Binaries
+
+Taskchampion-sync-server is a single binary that serves HTTP requests on a TCP
+port. The server does not implement TLS; for public deployments, the
+recommendation is to use a reverse proxy such as Nginx, haproxy, or Apache
+httpd.
+
+One binary is provided for each storage backend:
+
+- `taskchampion-sync-server` (SQLite)
+- `taskchampion-sync-server-postgres` (Postgres)
+
+### Running the Binary
+
+The server is configured with command-line options or environment variables.
+See the `--help` output for full details.
+
+For the SQLite binary, the `--data-dir` option or `DATA_DIR` environment
+variable specifies where the server should store its data. For the Postgres
+binary, the `--connection` option or `CONNECTION` environment variable
+specifies the connection information, in the form of a [LibPQ-style connection
+URI](https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING-URIS).
+The remaining options are common to all binaries.
+
+The `--listen` option specifies the interface and port the server listens on.
+It must contain an IP-Address or a DNS name and a port number. This option is
+mandatory, but can be repeated to specify multiple interfaces or ports. This
+value can be specified in environment variable `LISTEN`, as a comma-separated
+list of values.
+
+By default, the server will allow all clients and create them in the database
+on first contact. There are two ways to limit the clients the server will
+interact with:
+
+- To limit the accepted client IDs, specify them in the environment variable
+`CLIENT_ID`, as a comma-separated list of UUIDs. Client IDs can be specified
+with `--allow-client-id`, but this should not be used on shared systems, as
+command line arguments are visible to all users on the system. This convenient
+option is suitable for personal and small-scale deployments.
+
+- To disable the automatic creation of clients, use the `--no-create-clients`
+flag or the `CREATE_CLIENTS=false` environment variable. You are now
+responsible for creating clients in the database manually, so this option is
+more suitable for large scale deployments. See [Integration](../integration.md)
+for more information on such deployments.
+
+The server only logs errors by default. To add additional logging output, set
+environment variable `RUST_LOG` to `info` to get a log message for every
+request, or to `debug` to get more verbose debugging output.