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.

entrypoint-postgres.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+set -e
+echo "starting entrypoint script..."
+if [ "$1" = "/bin/taskchampion-sync-server-postgres" ]; then
+    : ${DATA_DIR:=/var/lib/taskchampion-sync-server/data}
+    export DATA_DIR
+    echo "setting up data directory ${DATA_DIR}"
+    mkdir -p "${DATA_DIR}"
+    chown -R taskchampion:users "${DATA_DIR}"
+    chmod -R 700 "${DATA_DIR}"
+
+    : ${LISTEN:=0.0.0.0:8080}
+    export LISTEN
+    echo "Listen set to ${LISTEN}"
+
+    if [ -n "${CLIENT_ID}" ]; then
+        export CLIENT_ID
+        echo "Limiting to client ID ${CLIENT_ID}"
+    else
+        unset CLIENT_ID
+    fi
+
+    if [ "$(id -u)" = "0" ]; then
+        echo "Running server as user 'taskchampion'"
+        exec su-exec taskchampion "$@"
+    fi
+else
+    eval "${@}"
+fi