workflows: update checkout action to current major version

Fixes deprecation warning for Node.js 12:

    https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

Author: bgilbert

.github/workflows/containers.yml

@@ -26,7 +26,7 @@ jobs:
       - run: sudo apt-get -qq update
       - name: Install libsystemd-dev
         run: sudo apt-get install libsystemd-dev
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Setup go
         uses: actions/setup-go@v1
         with: