Fix Docker workflow not pushing on manual trigger

Also fix issue where workflow fails on pull requests when the
user who made the pull request doesn't have the secrets set.
Also allow other workflows to be triggered manually.
Also don't trigger any workflows on pull requests to master, as
there shouldn't be any pull requests against master.

.github/workflows/main.yml

@@ -12,10 +12,12 @@ on:
       - 'doc/**'
       - 'images/**'
   pull_request:
-    branches: [ master, develop ]
+    branches: [ develop ]
     paths-ignore:
       - 'doc/**'
       - 'images/**'
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
 
 jobs:
   build: