feat: automatic releases (#165)

This uses:
    1) Renovate for managing the latest `thelounge` version.
    2) Release Please Action for automating creation of release PRs and
       releases (once the release PR is merged).
    3) Automerge Action for automatically merging PRs created by Release
       Please Action.

.github/workflows/automerge.yml

@@ -0,0 +1,34 @@
+name: automerge
+
+on:
+  pull_request:
+    types:
+      - labeled
+      - unlabeled
+      - synchronize
+      - opened
+      - edited
+      - ready_for_review
+      - reopened
+      - unlocked
+  pull_request_review:
+    types:
+      - submitted
+  check_suite:
+    types:
+      - completed
+  status: {}
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  automerge:
+    runs-on: ubuntu-latest
+    steps:
+      - id: automerge
+        name: automerge
+        uses: pascalgn/automerge-action@v0.15.6
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"

.github/workflows/release-please.yml

@@ -0,0 +1,25 @@
+name: Release
+
+on:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  release:
+    name: release
+    runs-on: ubuntu-latest
+    steps:
+      - uses: google-github-actions/release-please-action@v3
+        id: release
+        with:
+          release-type: simple
+          package-name: thelounge-docker
+          prerelease: true
+          include-v-in-tag: false
+          labels: |
+            automerge

renovate.json

@@ -0,0 +1,24 @@
+{
+    "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+    "automerge": true,
+    "platformAutomerge": true,
+    "automergeType": "pr",
+    "commitBody": "Release-As: {{newVersion}}",
+    "enabledManagers": ["regex"],
+    "regexManagers": [
+        {
+            "fileMatch": ["Dockerfile", "Makefile"],
+            "matchStrings": [
+                "THELOUNGE_VERSION\\??=(?<currentValue>[^\\s]+)"
+            ],
+            "datasourceTemplate": "npm",
+            "packageNameTemplate": "thelounge"
+        }
+    ],
+    "packageRules": [
+        {
+            "matchPackageNames": ["thelounge"],
+            "allowedVersions": "\\d+\\.\\d+\\.\\d+.*"
+        }
+    ]
+}