ci(backport): fetch original PR description

Backport PR will fetch the original PR description and other content and
amend the backport PR description with it.

.backportrc.json

@@ -2,6 +2,7 @@
   "repoOwner": "loft-sh",
   "repoName": "vcluster",
   "targetBranchChoices": ["v0.19", "v0.20"],
+    "prDescription": "Backport from `{{sourceBranch}}` to `{{targetBranch}}`\n\nOriginal PR Nr.: #{{sourcePullRequest.number}}\n\n### Backported Commits:\n{{#each commits}}\n- {{shortSha this.sourceCommit.sha}} {{this.sourceCommit.message}}\n{{/each}}\n\n## Original PR Description:\n{{sourcePullRequest}}",
   "branchLabelMapping": {
     "^backport-to-(.+)$": "$1"
   }

.github/workflows/backport.yaml

@@ -10,6 +10,34 @@ jobs:
     if: github.event.pull_request.merged == true && !(contains(github.event.pull_request.labels.*.name, 'backport'))
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install GH CLI
+        uses: dev-hanz-ops/[email protected]
+
+      - name: Fetch PR description and update backportrc
+        env:
+          GH_TOKEN: ${{ secrets.GH_ACCESS_TOKEN }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          REPO: ${{ github.repository }}
+        run: |
+          # Grab raw original PR body, gh CLI uses the default token.
+          BODY_CONTENT="$(gh api "/repos/$REPO/pulls/$PR_NUMBER" | jq -r .body)"
+
+          # Escape the markdown content so that sed can produce valid JSON.
+          # Remove starting and ending quites and replace new lines with literals.
+          ESCAPED_CONTENT=$(echo "$BODY_CONTENT" | jq -aRs .)
+          ESCAPED_CONTENT="${ESCAPED_CONTENT%\"}"
+          ESCAPED_CONTENT="${ESCAPED_CONTENT#\"}"
+          ESCAPED_CONTENT=$(echo "$ESCAPED_CONTENT" | sed 's/[\/&]/\\&/g')
+
+          # sourcePullRequest is an actual variable, but the PR description is not provided byt the action or the CLI.
+          # Instead we are using it as a substitution target and replacing it with the markdown content.
+          sed -i "s/{{sourcePullRequest}}/$ESCAPED_CONTENT/g" .backportrc.json
+
       - name: Backport Action
         uses: sorenlouv/[email protected]
         with: