Update docker file to better handle starting without an existing game folder

bin/unix/evennia-docker-start.sh

@@ -1,10 +1,18 @@
-#! /bin/bash
+#! /bin/sh
 
 # called by the Dockerfile to start the server in docker mode
 
 # remove leftover .pid files (such as from when dropping the container)
 rm /usr/src/game/server/*.pid >& /dev/null || true
 
-# start evennia server; log to server.log but also output to stdout so it can 
-# be viewed with docker-compose logs
-exec 3>&1; evennia start -l
+PS1="evennia|docker \w $ "
+
+cmd="$@"
+output="Docker starting with argument '$cmd' ..."
+if test -z $cmd; then
+    cmd="bash"
+    output="No argument given, starting shell ..."
+fi
+
+echo $output
+exec 3>&1; $cmd