diff --git a/.github/workflows/docker.yml b/.github/workflows/docker.yml
index cf7fde76f..c318da91e 100644
--- a/.github/workflows/docker.yml
+++ b/.github/workflows/docker.yml
@@ -1,9 +1,8 @@
+#
+# Build docker images
+#
 name: Docker Images
 
-env:
-  ctor_id: ${{ vars.DOCKER_ID }}
-  ctor_url: https://github.com/${{github.repository}}
-
 on:
   push:
     branches: [master]
@@ -15,7 +14,12 @@ concurrency:
   group: ${{ github.ref }}
   cancel-in-progress: true
 
+env:
+  ctor_id: ${{ vars.DOCKER_ID }}
+  ctor_url: https://github.com/${{github.repository}}
+
 jobs:
+  # Build the base-feature intermediate images (cached and not shipped).
   base:
     uses: ./.github/workflows/docker_prime.yml
     with:
@@ -26,6 +30,7 @@ jobs:
       machines: ${{vars.DOCKER_MACHINES}}
       test: ${{contains(github.events.push.commits[0].message, '[ci test]')}}
 
+  # Build the full-feature intermediate images (cached and not shipped).
   full:
     uses: ./.github/workflows/docker_prime.yml
     needs: [base]
@@ -37,6 +42,7 @@ jobs:
       machines: ${{vars.DOCKER_MACHINES}}
       test: ${{contains(github.events.push.commits[0].message, '[ci test]')}}
 
+  # Build the leaf images (shipped and not cached)
   built:
     needs: [base, full]
     runs-on: ${{matrix.machine}}
diff --git a/.github/workflows/docker_prime.yml b/.github/workflows/docker_prime.yml
index 06dd5be8c..537345e08 100644
--- a/.github/workflows/docker_prime.yml
+++ b/.github/workflows/docker_prime.yml
@@ -1,3 +1,9 @@
+#
+# Build intermediate images
+#
+# Called to build lower-layer images which other images depend on. These are
+# cached for use by the next layer but not shipped to users.
+#
 name: Docker Images Prime
 
 on:
@@ -5,32 +11,38 @@ on:
     inputs:
       id:
         type: string
+        description: Dockerhub acct/repo identity.
       url:
         type: string
+        description: Git repository for checkout.
       features:
         type: string
+        description: JSON array of feature-set names to build images for.
       distros:
         type: string
+        description: JSON array of operating system distros to build for.
       machines:
         type: string
+        description: JSON array of machines to build for.
       test:
         type: boolean
         default: false
         required: false
+        description: Echo all docker commands rather than invoking them.
+
+concurrency:
+  group: ${{github.workflow}}-${{inputs.features}}
+  cancel-in-progress: false
 
 env:
   ctor_id: ${{inputs.id}}
   ctor_url: ${{inputs.url}}
 
-concurrency:
-  group: ${{github.run_id}}
-  cancel-in-progress: true
-
 jobs:
   prime:
     runs-on: ${{matrix.machine}}
     strategy:
-      fail-fast: false
+      fail-fast: true
       matrix:
         feature: ${{fromJSON(inputs.features)}}
         distro: ${{fromJSON(inputs.distros)}}