feat(infra): Added way to run custom commands for different services

- new `EXTRA_FILES` docker build args to specify directory to be copied at
  /extra
- run `${SERVICE}/entrypoint.sh` as docker entry point
- added src/empty folder to so if EXTRA_FILES isn't set => use empty
  folder (since no conditional COPY)

src/icons/entrypoint.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+set -e
+set -x
+# do anything here
+
+cp -r /extra /files
+
+# run the CMD [ ... ] from the dockerfile
+exec "$@"