feat: add GHA workflow to keep track of the Ubuntu image SHA

* This is to workaround the 3mo timeout for GHA scheduled actions on repos with
  no new commits, by automatically creating new commits.

.github/workflows/update-ubuntu-sha.yaml

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+name: Update SHA of latest ubuntu images
+# This is to workaround the 3mo timeout for GHA scheduled actions on repos with 
+# no new commits, by automatically creating new commits.
+on:
+  schedule:
+    # check for a new ubuntu image every week at 04:17am on mondays
+    - cron: "17 04 * * 1"
+  # let us manually trigger a build
+  workflow_dispatch:
+env:
+  BASE_IMAGE_NAME: base-docker
+  ACTION_IMAGE_NAME: base-action
+jobs:
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@master
+    - uses: extractions/setup-just@69d82fb0233557aec017ef13706851d0694e0f1d
+    - name: update SHAs
+      run: just update-docker-shas
+    - name: Commit file
+      run: |
+        git status
+        git add *.sha
+        if git diff-index --quiet HEAD; then
+          exit
+        fi
+        git config --local user.email "[email protected]"
+        git config --local user.name "GitHub Action"
+        git commit -m "Update base image SHA files"
+        git push origin

justfile

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+
+default:
+  @just --list
+
+# Update the files tracking the SHAs of ubuntu docker image
+update-docker-shas:
+  @just _update-sha "ubuntu:20.04"
+  @just _update-sha "ubuntu:22.04"
+
+_update-sha os:
+  echo {{ os }}
+  docker image pull {{ os }}
+  docker inspect --format='{{{{index .RepoDigests 0}}' {{ os }} > {{ os }}.sha

ubuntu:20.04.sha

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+ubuntu@sha256:33a5cc25d22c45900796a1aca487ad7a7cb09f09ea00b779e3b2026b4fc2faba

ubuntu:22.04.sha

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+ubuntu@sha256:aabed3296a3d45cede1dc866a24476c4d7e093aa806263c27ddaadbdce3c1054