docker: update following name changes:

`backend_e2e` now print properly backend logs

docker/prosody-e2e/entrypoint.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+set -e
+
+usermod -u "$(stat -c %u /var/lib/prosody/.)" prosody
+
+if [[ "$1" != "prosody" ]]; then
+    exec prosodyctl "$@"
+    exit 0;
+fi
+
+if [ "$LOCAL" -a  "$PASSWORD" -a "$DOMAIN" ] ; then
+    prosodyctl register "$LOCAL" "$DOMAIN" "$PASSWORD"
+fi
+
+exec runuser -u prosody -- "$@"