Add: [Actions] preview workflow, where PRs can be reviewed in the browser

When a developer attaches the "preview" label, a build is created
and published on https://preview.openttd.org/. After that, new
pushes to the PR are automatically build (as long as the "preview"
label exists).

If a non-developer attaches the "preview" label, it will be
removed.

.github/workflows/preview_push.yml

@@ -0,0 +1,66 @@
+name: Preview push
+
+on:
+  pull_request_target:
+    types:
+    - synchronize
+
+jobs:
+  check_new_preview:
+    name: Check preview needs update
+
+    runs-on: ubuntu-20.04
+
+    steps:
+    - name: Check if earlier preview exists
+      id: earlier_preview
+      uses: octokit/request-action@v2.x
+      with:
+        route: GET /repos/{owner}/{repo}/deployments
+        owner: ${{ github.event.repository.owner.login }}
+        repo: ${{ github.event.repository.name }}
+        environment: preview-pr-${{ github.event.number }}
+        per_page: 1
+      env:
+        GITHUB_TOKEN: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+
+    - if: toJson(fromJson(steps.earlier_preview.outputs.data)) != '[]'
+      name: Check for preview label
+      id: preview_label
+      uses: octokit/request-action@v2.x
+      with:
+        route: GET /repos/{owner}/{repo}/issues/{issue_number}/labels
+        owner: ${{ github.event.repository.owner.login }}
+        repo: ${{ github.event.repository.name }}
+        issue_number: ${{ github.event.number }}
+      env:
+        GITHUB_TOKEN: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+
+    - if: toJson(fromJson(steps.earlier_preview.outputs.data)) != '[]' && contains(fromJson(steps.preview_label.outputs.data).*.name, 'preview')
+      name: Create deployment
+      id: deployment
+      uses: octokit/request-action@v2.x
+      with:
+        route: POST /repos/{owner}/{repo}/deployments
+        mediaType: |
+          previews:
+          - ant-man
+          - flash
+        owner: ${{ github.event.repository.owner.login }}
+        repo: ${{ github.event.repository.name }}
+        ref: ${{ github.event.pull_request.head.sha }}
+        task: deploy:preview
+        auto_merge: false
+        required_contexts: "[]"
+        environment: preview-pr-${{ github.event.number }}
+        description: "Preview for Pull Request #${{ github.event.number }}"
+      env:
+        GITHUB_TOKEN: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+
+    - if: toJson(fromJson(steps.earlier_preview.outputs.data)) != '[]' && contains(fromJson(steps.preview_label.outputs.data).*.name, 'preview')
+      name: Trigger 'preview build'
+      uses: peter-evans/repository-dispatch@v1
+      with:
+        token: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+        event-type: "Preview build #${{ github.event.number }}"
+        client-payload: '{"folder": "pr${{ github.event.number }}", "sha": "${{ github.event.pull_request.head.sha }}", "deployment_id": "${{ fromJson(steps.deployment.outputs.data).id }}"}'