Allow specifying client ID when running docker-compose (#101)

This also fixes up some handling of default values in the entrypoint.

docker-entrypoint.sh

@@ -2,12 +2,28 @@
 set -e
 echo "starting entrypoint script..."
 if [ "$1" = "/bin/taskchampion-sync-server" ]; then
-    echo "setting data directories"
+    : ${DATA_DIR:=/var/lib/taskchampion-sync-server}
+    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 "switching to user 'taskchampion'"
+        echo "Running server as user 'taskchampion'"
         exec su-exec taskchampion "$@"
     fi
+else
+    eval "${@}"
 fi