.github: drop branches filter with single asterisk from workflows (#17682)

Drop usage of the branches filter with a single asterisk as this matches
against zero or more characters but not a forward slash, resulting in
PRs to branch names with forwards slashes in them not having these
workflow run against them as expected.

Updates https://github.com/tailscale/corp/issues/33523

Signed-off-by: Mario Minardi <mario@tailscale.com>

.github/workflows/docker-file-build.yml

@@ -4,8 +4,6 @@ on:
     branches:
     - main
   pull_request:
-    branches:
-      - "*"
 jobs:
   deploy:
     runs-on: ubuntu-latest