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

Change-Id: I2362073eba6df29db1ab1f054621805eb29429fd
diff --git a/Dockerfile b/Dockerfile
index 0409463..c266881 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -9,6 +9,7 @@
     # 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