[MISC] Add github comment workflow for automated builds (#605)

.github/workflows/prcomments.md

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+# Containers
+
+These changes can be tested with Docker at the following location:
+
+#### Local Branch
+* docker pull authelia/authelia:{branch_name}
+
+#### Public Repo PR
+* docker pull authelia/authelia:PR{pull_id}
+
+Public Repo PR's will need to be approved by team member prior to it being deployed in the pipeline.

.github/workflows/prcomments.yml

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+name: Add container pull commands
+on: pull_request
+jobs:
+  comment:
+    name: Add container pull commands
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - uses: harupy/comment-on-pr@master
+        env:
+          GITHUB_TOKEN: ${{ secrets.COMMENT_ON_PR_GH_TOKEN }}
+        with:
+          filename: prcomments.md