Add a binary that uses a Postgres backend

Building of this binary is controlled with features, allowing downstream
users to build just the SQLite version and not be concerned with the
tokio-postgres dependency tree (which includes links to OpenSSL and
other details). The Postgres version is disabled by default.

This does not change the binary name for the SQLite build, just to avoid
confusion for people upgrading to the new version.

README.md

@@ -17,7 +17,7 @@ for more on how to use this project.
 
 ## Repository Guide
 
-The repository is comprised of three crates:
+The repository is comprised of four crates:
 
  - `taskchampion-sync-server-core` implements the core of the protocol
  - `taskchampion-sync-server-storage-sqlite` implements an SQLite backend for the core
@@ -60,16 +60,34 @@ cargo build --release
 After build the binary is located in
 `target/release/taskchampion-sync-server`.
 
-### Building the Container
+#### Building the Postgres backend
 
-To build the container, execute the following commands.
+The storage backend is controlled by Cargo features `postres` and `sqlite`.
+By default, only the `sqlite` feature is enabled.
+To enable building the Postgres backend, add `--features postgres`.
+The Postgres binary is located in
+`target/release/taskchampion-sync-server-postgres`.
 
+### Building the Docker Images
+
+To build the images, execute the following commands.
+
+SQLite:
 ```sh
 source .env
 docker build \
   --build-arg RUST_VERSION=${RUST_VERSION} \
   --build-arg ALPINE_VERSION=${ALPINE_VERSION} \
-  -t taskchampion-sync-server .
+  -t taskchampion-sync-server docker/sqlite
+```
+
+Postgres:
+```sh
+source .env
+docker build \
+  --build-arg RUST_VERSION=${RUST_VERSION} \
+  --build-arg ALPINE_VERSION=${ALPINE_VERSION} \
+  -t taskchampion-sync-server-postgres docker/postgres
 ```
 
 Now to run it, simply exec.