feat: add basic initial backport

Signed-off-by: Alan Clucas <alan@clucas.org>

.github/workflows/backport.yaml

@@ -0,0 +1,40 @@
+name: Backport merged pull request
+on:
+  pull_request_target:
+    types: [closed]
+  issue_comment:
+    types: [created]
+permissions:
+  contents: write # so it can comment
+  pull-requests: write # so it can create pull requests
+jobs:
+  backport:
+    name: Backport pull request
+    runs-on: ubuntu-latest
+
+    # Only run when pull request is merged
+    # or when a comment starting with `/backport` is created by someone other than the
+    # https://github.com/backport-action bot user (user id: 97796249). Note that if you use your
+    # own PAT as `github_token`, that you should replace this id with yours.
+    if: >
+      (
+        github.event_name == 'pull_request_target' &&
+        github.event.pull_request.merged
+      ) || (
+        github.event_name == 'issue_comment' &&
+        github.event.issue.pull_request &&
+        github.event.comment.user.id != 97796249 &&
+        startsWith(github.event.comment.body, '/backport')
+      )
+    steps:
+      - uses: actions/checkout@v4
+      - name: Create backport pull requests
+        uses: korthout/backport-action@be567af183754f6a5d831ae90f648954763f17f5 # v3.1.0
+        with:
+          label_pattern: ^backport/([^ ]+)$
+          pull_title: ${pull_title} [Backport ${target_branch}]
+          pull_description: |-
+            Automated backport of #${pull_number} to `${target_branch}`
+            Associated issues if any: [${issue_refs}]
+            * [ ] Good to test
+          conflict_resolution: draft_commit_conflicts

.github/workflows/ci-build.yaml

@@ -8,6 +8,7 @@ on:
   pull_request:
     branches:
       - "main"
+      - "release-*"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}

.github/workflows/docs.yaml

@@ -7,6 +7,7 @@ on:
   pull_request:
     branches:
       - main
+      - release/*
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}

.spelling

@@ -139,7 +139,9 @@ args
 async
 auth
 backend
+backport
 backported
+backporting
 boolean
 booleans
 buildkit

docs/releasing.md

@@ -1,5 +1,20 @@
 # Release Instructions
 
+## Backporting fixes
+
+Fixes can be backported using the automated cherry-picker.
+This will only work on merged commits, and always chooses the branches to backport based on labels.
+Label of the form `backport/<targetbranch>` will cherry-pick this commit to the given branch - e.g. `backport/release-3.6`.
+
+The backport will be triggered on merging, or on by adding a comment starting with `/backport` - any extra text is ignored by the bot.
+You cannot specify the backport target using comment text, only a label.
+
+Backported commits can be backported, so it may make sense to backport the `release-3.6` backport to `release-3.5` for example.
+
+Backported fixes may fail on merge conflicts and assistance will be given by the bot on how to manually create a backport.
+
+Commits generated this way are merged in the normal way, this part is not automated.
+
 ## Cherry-Picking Fixes
 
 ✋ Before you start, make sure you have created a release branch (e.g. `release-3.3`) and it's passing CI.