gha: split link-check to separate job

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.github/workflows/build-pr.yml

@@ -14,6 +14,15 @@ jobs:
       - uses: actions/checkout@v2
       - name: build image
         run: docker build --target=current -t documentation:latest .
+  validate:
+    name: validate links
+    runs-on: ubuntu-18.04
+    env:
+      DOCKER_BUILDKIT: '1'
+    steps:
+      - name: print docker info
+        run: docker version && docker info
+      - uses: actions/checkout@v2
       - name: copy files to host
         run: docker build --target=deploy-source --output=./_site .
       - name: check for broken links