Actions: Revert to old stable docker image regex (#1952)

Technical difficulties with new tag-based image releases necessitates
this PR which changes the regex for the stable docker image workflow
file to an earlier value

.github/workflows/stable-docker.yml

@@ -3,7 +3,7 @@ name: Stable Docker images
 on:
   push:
     tags:
-      - "v[0-9]+.[0-9]+.[0-9]+[a-zA-Z]*"
+      - "v[0-9]+.[0-9]+.[0-9]+"
 
 jobs:
   docker-build: