add wrapper script for a clean startup in container

wait-for-synapse.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+# wait-for-synapse.sh
+#
+# based on an exmaple from https://docs.docker.com/compose/startup-order/
+#
+# command: ["./wait-for-synapse.sh", "synapse", "python", "/usr/src/app/pnut-matrix.py", "-d"]
+
+set -e
+
+host="$1"
+# Shift arguments with mapping:
+# - $0 => $0
+# - $1 => <discarded>
+# - $2 => $1
+# - $3 => $2
+# - ...
+# This is done for `exec "$@"` below to work correctly
+shift
+
+until [ "200" == $(curl -s -o /dev/null --head -w "%{http_code}" https://${host}/_matrix/client/versions) ]; do
+    >&2 echo "synapse is unavailable - sleeping"
+    sleep 3
+done
+
+>&2 echo "synapse is up - executing command"
+exec "$@"