ci: change Docker workflow to run twice a month and on tag pushes

Change-Id: Id3528e537be00fc7d24cb0e4d7868862343bb6e5
diff --git a/Dockerfile b/Dockerfile
index 9d73d0b..14a5c65 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -21,6 +21,7 @@
     # use 'apt-get distclean' when we upgrade to ubuntu:24.04
     && rm -rf /var/lib/apt/lists/*
 
+ARG JOBS
 RUN --mount=type=bind,rw,target=/src <<EOF
 set -eux
 cd /src