ENH: Add Github workflow to update the "nightly-main" branch daily

Introduce a new workflow that updates the 'nightly-main' preview branch
daily at 04:00 UTC. This scheduled workflow ensures it runs after Kitware-hosted nightly
builds, aligned with the dashboards' 11:00 PM ET trigger time.

.github/workflows/update-slicer-preview-branch.yml

@@ -0,0 +1,91 @@
+name: Update Preview Branch
+
+on:
+  schedule:
+    # Daily at 04:00 AM UTC (this corresponds to the time set in CTestConfig.cmake + 1 hour)
+    #
+    # This workflow is synchronized with the nightly builds on the Kitware-hosted dashboards,
+    # which are configured via crontab to trigger daily at 11:00 PM Eastern Time (ET). The conversion
+    # from ET to UTC depends on whether daylight saving time is in effect:
+    #
+    # - During the spring/summer months (typically March to November), Eastern Daylight Time (EDT),
+    #   which is UTC-04:00, is observed. Thus, 11:00 PM EDT corresponds to 03:00 AM UTC.
+    # - During the autumn/winter months (typically November to March), Eastern Standard Time (EST),
+    #   which is UTC-05:00, is observed. Thus, 11:00 PM EST corresponds to 04:00 AM UTC.
+    #
+    # The build jobs are configured to start via the crontab on the Kitware-hosted dashboards at
+    # 11:00 PM ET, and this workflow is scheduled to run daily at 04:00 AM UTC to ensure it executes
+    # after the nightly builds have started and completed their source checkout, regardless of daylight
+    # saving time.
+    #
+    # This is important because the preview branch should be updated to reflect the state of the
+    # repository as of the latest nightly build.
+    - cron: "0 4 * * *"
+  # Manual trigger
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  update-slicer-preview-branch:
+    permissions:
+      # Needed in publish step to update the preview branch
+      contents: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        with:
+          ref: main
+          # The 'fetch-depth' option ensures that a sufficient number of commits are retrieved (250)
+          # to allow the "git rev-parse" command used below to accurately reference a commit based
+          # on time.
+          # This mitigates potential warnings that occur when there aren't enough commits fetched,
+          # such as:
+          #   warning: log for 'main' only goes back to ...
+          # We assume no more than 250 commits are made in a single day.
+          # fetch-depth: 250
+
+      - name: Install time zone and daylight-saving time data
+        run: |
+          sudo apt-get -y install tzdata
+        env:
+          DEBIAN_FRONTEND: noninteractive
+
+      - name: "Convert dashboard start time from ET to UTC"
+        id: convert
+        run: |
+          # time=$(TZ=":US/Eastern" date -d"2024-09-20 23:00:00"  "+%H:%M:%S %z")
+          time=$(TZ=":US/Eastern" date -d"2024-09-20 23:00:00"  "+%Y%m%d %H:%M:%S %z")
+          echo "time [$time]"
+          echo "time=$time" >> $GITHUB_OUTPUT
+
+      - name: "Retrieve preview branch SHA"
+        id: retrieve
+        run: |
+          git log -n3 origin/main
+          sha=$(TZ=":US/Eastern" git rev-list -1 --before="${TIME}" main)
+          echo "sha [$sha]"
+          echo "sha=$sha" >> $GITHUB_OUTPUT
+        env:
+          TIME: ${{ steps.convert.outputs.time }}
+
+      - uses: actions/create-github-app-token@5d869da34e18e7287c1daad50e0b8ea0f506ce69 # v1.11.0
+        id: app-token
+        with:
+          app-id: ${{ vars.SLICER_APP_ID }}
+          private-key: ${{ secrets.SLICER_APP_PRIVATE_KEY }}
+
+      - name: "Publish"
+        run: |
+          if [ -z "$SHA" ]; then
+            echo "::error ::Failed to retrieve SHA"
+            exit 1
+          fi
+          remote=https://${SLICERBOT_GITHUB_TOKEN}@github.com/${REPOSITORY}.git
+          git push $remote $SHA:refs/heads/${PREVIEW_BRANCH} --force
+        env:
+          SHA: ${{ steps.retrieve.outputs.sha }}
+          SLICERBOT_GITHUB_TOKEN: ${{ steps.app-token.outputs.token }}
+          REPOSITORY: ${{ github.repository }}
+          PREVIEW_BRANCH: nightly-main

CTestConfig.cmake

@@ -1,4 +1,8 @@
 set(CTEST_PROJECT_NAME "Slicer")
+
+# If the nightly start time is updated, ensure the corresponding change is reflected
+# in ".github/workflows/update-slicer-preview-branch.yml" and in the crontab or task scheduler
+# of the Kitware-hosted dashboards to maintain synchronization across all systems.
 set(CTEST_NIGHTLY_START_TIME "3:00:00 UTC")
 
 if(NOT DEFINED CDASH_PROJECT_NAME)