Add bashbrew as the entrypoint to make using the image easier

bashbrew-entrypoint.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+set -e
+
+if [ "${1:0:1}" = '-' ]; then
+	set -- bashbrew "$@"
+fi
+
+# if our command is a valid bashbrew subcommand, let's invoke it through bashbrew instead
+# (this allows for "docker run bashbrew build", etc)
+if bashbrew "$1" --help > /dev/null 2>&1; then
+	set -- bashbrew "$@"
+fi
+
+exec "$@"