Harden GitHub actions

.github/dependabot.yml

@@ -0,0 +1,22 @@
+version: 2
+updates:
+  - package-ecosystem: bundler
+    directory: "/"
+    schedule:
+      interval: weekly
+    cooldown:
+      semver-major-days: 7
+      semver-minor-days: 3
+      semver-patch-days: 2
+      default-days: 7
+
+  - package-ecosystem: github-actions
+    directory: "/"
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+    schedule:
+      interval: weekly
+    cooldown:
+      default-days: 7

.github/workflows/ruby.yml

@@ -1,10 +1,18 @@
 name: CI
 
-on: [ push, pull_request ]
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    types: [ opened, synchronize ]
+
+permissions: {}
 
 jobs:
   tests:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     strategy:
       fail-fast: false
       matrix:
@@ -25,15 +33,18 @@ jobs:
             rubygems_version: '3.6.9'
     name: Ruby ${{ matrix.ruby }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2.7.0
+        with:
+          persist-credentials: false
       - name: Set up Ruby
-        uses: ruby/setup-ruby@v1
+        uses: ruby/setup-ruby@afeafc3d1ab54a631816aba4c914a0081c12ff2f # v1.310.0
         with:
           ruby-version: ${{ matrix.ruby }}
       - name: Update RubyGems
         env:
           RUBYGEMS_VERSION: ${{ matrix.rubygems_version }}
         run: |
+            # shellcheck disable=SC2086 # empty version is intentional: no arg means update to latest
             gem update --system ${RUBYGEMS_VERSION:-}
             gem -v
       - name: Install dependencies
@@ -44,13 +55,36 @@ jobs:
   # rubocop linting
   rubocop:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2.7.0
+        with:
+          persist-credentials: false
       - name: Set up Ruby
-        uses: ruby/setup-ruby@v1
+        uses: ruby/setup-ruby@afeafc3d1ab54a631816aba4c914a0081c12ff2f # v1.310.0
         with:
           ruby-version: 2.7
       - name: Install dependencies
         run: bundle install --jobs 4 --retry 3
       - name: Run rubocop
         run: bundle exec rubocop --parallel
+
+  lint-actions:
+    name: GitHub Actions audit
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+
+    steps:
+      - uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2.7.0
+        with:
+          persist-credentials: false
+
+      - name: Run actionlint
+        uses: rhysd/actionlint@393031adb9afb225ee52ae2ccd7a5af5525e03e8 # v1.7.11
+
+      - name: Run zizmor
+        uses: zizmorcore/zizmor-action@71321a20a9ded102f6e9ce5718a2fcec2c4f70d8 # v0.5.2
+        with:
+          advanced-security: false