workflow: don't push Docker container on PR (#2984)

The docker container on PR action is broken in case the PR is not opened
within the Gluon project.

Remove the build on PR so the pipeline does not fail for third party
PRs.

Signed-off-by: David Bauer <mail@david-bauer.net>

.github/workflows/build-container.yml

@@ -7,9 +7,6 @@ on:
       - 'master'
     tags:
       - 'v*'
-  pull_request:
-    branches:
-      - 'master'
 
 env:
   REGISTRY: ghcr.io