Introduce a github action which builds a cockpit preview when requested

It currently reacts to /livetest comments on PRs and builds cockpit from
the PR's checkout. Then serves it on logs.cockpit-project.org on a port
specified on a follow-up comment to the /livetest.

Cockpit developers can now utilize this workflow to review a PR without
having to pull and compile the code locally.

.github/workflows/livetest.yml.disabled

@@ -0,0 +1,101 @@
+name: Live test cockpit from PRs
+# Triggered when a repository collaborator comments /livetest on a PR
+# This is currently configured to run on cockpit-project org.
+# If your project lives somewhere else than the github cockpit-project organization,
+# you need to set up your own relay host, add a secret with the private SSH key, and
+# adjust the RELAY_HOST env variable and SINK_SSHRELAY_PRIVATE secret name.
+
+on:
+  issue_comment:
+    types: [created]
+
+jobs:
+  pr-info:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Query author repository permissions
+        uses: octokit/request-action@v2.x
+        id: user_permission
+        with:
+          route: GET /repos/${{ github.repository }}/collaborators/${{ github.event.sender.login }}/permission
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      # restrict /livetest to users with admin or write permission for the repository
+      # see https://docs.github.com/en/free-pro-team@latest/rest/reference/repos#get-repository-permissions-for-a-user
+      # store output if user is allowed in allowed_user job output so it has to be checked in downstream job
+      - name: Check if user does have correct permissions
+        if: contains('admin write', fromJson(steps.user_permission.outputs.data).permission)
+        id: check_user_perm
+        run: |
+          echo "User '${{ github.event.sender.login }}' has permission '${{ fromJson(steps.user_permission.outputs.data).permission }}' allowed values: 'admin', 'write'"
+          echo "::set-output name=allowed_user::true"
+
+    outputs:
+      allowed_user: ${{ steps.check_user_perm.outputs.allowed_user }}
+
+  live-test:
+    needs: pr-info
+    if: needs.pr-info.outputs.allowed_user == 'true' && github.event.issue.pull_request && startsWith(github.event.comment.body, '/livetest')
+    runs-on: ubuntu-20.04
+    env:
+      RELAY_HOST: logs.cockpit-project.org
+    steps:
+      - name: Install dependencies
+        run: |
+          sudo apt update
+          sudo apt install -y cockpit-system cockpit-ws nodejs npm sassc make
+
+      - name: Clone repository
+        uses: actions/checkout@v2
+        with:
+          # need this to also fetch tags
+          fetch-depth: 0
+
+      - name: Compile and install the project
+        run: make && sudo make install
+
+      - name: Create a new test user
+        # WARNING: if you want to add this users to sudoers make sure to propertly delete the SINK_SSHRELAY_PRIVATE
+        run: sudo useradd --create-home livetest; echo livetest:foobar | sudo chpasswd
+
+      - name: Get the commit SHA that triggered the workflow
+        id: get_commit_sha
+        run: |
+          echo "::set-output name=sha_short::$(git rev-parse --short HEAD)"
+
+      - name: Get ssh gateway port to forward to
+        id: remote_port
+        env:
+          SHA_SHORT: "${{ steps.get_commit_sha.outputs.sha_short }}"
+        run: |
+            echo ::set-output name=SSH_GATEWAY_PORT::$((16#$SHA_SHORT % 32768 + 1500))
+
+      - name: Write a comment on the PR with link to the cockpit instance
+        uses: actions/github-script@v3
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            github.issues.createComment({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: 'Cockpit preview: https://${{ env.RELAY_HOST }}:${{ steps.remote_port.outputs.SSH_GATEWAY_PORT }}'
+            })
+
+      - name: Create an SSH private key from the secret
+        run: |
+          sudo touch /run/SINK_SSHRELAY_PRIVATE && sudo chown $(id -u):$(id -g) /run/SINK_SSHRELAY_PRIVATE
+          echo "${{ secrets.SINK_SSHRELAY_PRIVATE }}" > /run/SINK_SSHRELAY_PRIVATE
+          chmod 600 /run/SINK_SSHRELAY_PRIVATE
+          mkdir -p ~/.ssh &&  ssh-keyscan -H ${{ env.RELAY_HOST }} > ~/.ssh/known_hosts
+
+      - name: Forward cockpit port 9090 to a server we can access
+        run: ssh -f -R ':${{ steps.remote_port.outputs.SSH_GATEWAY_PORT }}':localhost:9090 -i /run/SINK_SSHRELAY_PRIVATE -o ExitOnForwardFailure=yes sshgw@${{ env.RELAY_HOST }} 'sleep infinity'
+
+      - name: Block workflow from finishing
+        run: sleep 4h
+
+      - name: Cleanup ssh process
+        if: ${{ always() }}
+        run: pkill ssh || true