Build docker image on PRs (#36)

Author: mfisher87

.github/workflows/build-and-publish-docker-image.yml

@@ -4,9 +4,11 @@ on:
   push:
     branches:
       - "main"
-    paths:
+    paths: &paths
       - "docker/*"
       - ".github/workflows/build-and-publish-docker-image.yml"
+  pull_request:
+    paths: *paths
 
 
 permissions:
@@ -53,6 +55,8 @@ jobs:
         REPO_DIR: "/srv/repo"
         # Fix man pages that are disabled by Ubuntu 18 minimal base image
         # APPENDIX_FILE: "appendix"
+        # Skip pushing for PRs:
+        NO_PUSH: "${{ github.event_name == 'pull_request' }}"
 
     # Lets us monitor disks getting full as images get bigger over time
     - name: "Show how much disk space is left"