Add GitHub Actions workflow to publish releases

Add a workflow that will trigger on pushes to master which:
* Builds the HTML version of the spec (using the Makefile) in a build
  directory
* Moves the built HTML file as index.html to a versioned subdirectory
  of build
* Moves the built HTML file as index.html to a 'latest' subdirectory
  of build
* Switches to the 'gh-pages' branch and moves the conents of the build
  directory into the repo root
* Builds an index of published versions as index.html at the root of the
  repo root
* Commits the generated HTML files and pushes them to the gh-pages branch
  on GitHub

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

.github/workflows/release.yml

@@ -0,0 +1,81 @@
+name: Release specification
+on:
+  workflow_dispatch:
+  push:
+    branches: master
+
+jobs:
+  make-release:
+    name: Make and publish spec release
+    runs-on: ubuntu-latest
+    steps:
+    - name: Set up Python
+      uses: actions/setup-python@3105fb18c05ddd93efea5f9e0bef7a03a6e9e7df
+      with:
+        python-version: 3.x
+
+    - name: Find pip cache dir
+      id: pip-cache
+      run: echo "::set-output name=dir::$(pip cache dir)"
+
+    - name: pip cache
+      uses: actions/cache@26968a09c0ea4f3e233fdddbafd1166051a095f6
+      with:
+        # Use the os dependent pip cache directory found above
+        path: ${{ steps.pip-cache.outputs.dir }}
+        # A match with 'key' counts as cache hit
+        key: ${{ runner.os }}-pip-
+
+    - name: Clone main
+      uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f
+      with:
+        fetch-depth: 0
+
+    - name: Get previous version
+      id: prevver
+      run: |
+        prev_version=`git tag | sort -V -r | head -n 1 | cut -c 2-`
+        echo "::set-output name=prev_version::$(echo -n $prev_version)"
+
+    - name: Get version
+      id: getver
+      run: |
+        spec_version=`grep -oP 'VERSION \K(\d+\.\d+\.\d+)' tuf-spec.md`
+        echo "::set-output name=spec_version::$(echo -n $spec_version)"
+
+    - name: Make release
+      if: steps.getver.outputs.spec_version != steps.prevver.outputs.prev_version
+      uses: actions/create-release@0cb9c9b65d5d1901c1f53e5e66eaf4afd303e70e
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        tag_name: v${{ steps.getver.outputs.spec_version }}
+        release_name: v${{ steps.getver.outputs.spec_version }}
+        body: Specification release v${{ steps.getver.outputs.spec_version }}
+
+    - name: Build specification
+      if: steps.getver.outputs.spec_version != steps.prevver.outputs.prev_version
+      run: |
+        python -m pip install bikeshed
+        mkdir build && cd build
+        make -f ../Makefile release
+
+    - name: Switch branch
+      if: steps.getver.outputs.spec_version != steps.prevver.outputs.prev_version
+      uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f
+      with:
+        ref: gh-pages
+        clean: false
+
+    - name: Push generated specification
+      if: steps.getver.outputs.spec_version != steps.prevver.outputs.prev_version
+      run: |
+        git config user.name "TUF Specification Automation"
+        git config user.email [email protected]
+        rm -fr latest
+        mv build/* .
+        rmdir build
+        make index
+        git add .
+        git commit -m "Publish latest specification"
+        git push