Use Khronos asciidoctor-spec Docker image in CI

Per discussion with $bashbaug

N.b. at present the CI script has less parallelism than it could, at
least as I understand Actions. Some of the 'steps' could be split off
into 'jobs'. Might try that next once the basic build is working.

.github/workflows/presubmit.yml

@@ -3,30 +3,41 @@ name: Presubmit
 permissions:
   contents: read
 
-on: [push, pull_request]
+# Controls when the action will run.
+on:
+  # Triggers the workflow on push or manual dispatch
+  push:
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+  # When a pull request is opened from a local branch or fork
+  pull_request:
 
 jobs:
   build:
     name: Build all specs
     runs-on: ubuntu-latest
+    # We now refer to the container by its SHA instead of the name, to prevent
+    # caching problems when updating the image.
+    # container: khronosgroup/docker-images:asciidoctor-spec
+    container: khronosgroup/docker-images@sha256:bd30a83285a2ea062598f053b5bd8ebc843e16c639c0e4cd88ab4bbb4e63ead3
 
     steps:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
           submodules: recursive
 
-      - name: Install required packages
-        run: |
-          sudo apt-get install -y libpango1.0-dev libwebp-dev ghostscript fonts-lyx jing libavalon-framework-java libbatik-java python3-pyparsing
-          sudo gem install asciidoctor -v 2.0.16
-          sudo gem install coderay -v 1.1.1
-          sudo gem install rouge -v 3.19.0
-          sudo gem install ttfunk -v 1.7.0
-          sudo gem install hexapdf -v 0.27.0
-          sudo gem install asciidoctor-pdf -v 2.3.4
-          sudo gem install asciidoctor-mathematical -v 0.3.5
-          sudo pip install pyparsing
+      # - name: Install required packages
+      #   run: |
+      #     sudo apt-get install -y libpango1.0-dev libwebp-dev ghostscript fonts-lyx jing libavalon-framework-java libbatik-java python3-pyparsing
+      #     sudo gem install asciidoctor -v 2.0.16
+      #     sudo gem install coderay -v 1.1.1
+      #     sudo gem install rouge -v 3.19.0
+      #     sudo gem install ttfunk -v 1.7.0
+      #     sudo gem install hexapdf -v 0.27.0
+      #     sudo gem install asciidoctor-pdf -v 2.3.4
+      #     sudo gem install asciidoctor-mathematical -v 0.3.5
+      #     sudo pip install pyparsing
 
       - name: List git tag
         run: |
@@ -35,7 +46,7 @@ jobs:
       - name: Generate core specs (HTML and PDF)
         run: |
           python3 makeSpec -clean -spec core OUTDIR=out.core -j 5 api c env ext cxx4opencl
-          
+
       - name: Generate core + extension specs (HTML)
         run: |
           python3 makeSpec -clean -spec khr OUTDIR=out.khr -j 12 html