ci: one concurrency group for every workflow; a runner-image rebuild blocks the rest

Captain Bob, 2026-11-07: "I also want that rebuilds block other jobs
always." acceptance, nightly and release shared the group qemu; now
runner-image, build and nightly-docs do too, so a rebuild of the runner
image (on the host's Docker daemon) never overlaps another job, whatever
the runner's capacity (#121). On this server every CI run is then one at a
time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2

.gitea/workflows/acceptance.yml

@@ -18,7 +18,8 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: qemu                     # one QEMU at a time on the runner: shared with nightly.yml and release.yml
+  group: qemu                     # every workflow shares this one group: one run at a time,
+                                  # a runner-image rebuild included (Captain Bob, 2026-11-07)
   cancel-in-progress: false
 
 jobs:

.gitea/workflows/build.yml

@@ -13,6 +13,10 @@ on:
                                      # the server the wikis share (Captain Bob, 2026-11-07)
   workflow_dispatch:
 
+concurrency:
+  group: qemu                     # every workflow shares this one group (Captain Bob, 2026-11-07: a
+  cancel-in-progress: false       # runner-image rebuild blocks other jobs, always): one run at a time
+
 jobs:
   image:
     name: Image ${{ matrix.board }}

.gitea/workflows/nightly-docs.yml

@@ -14,6 +14,10 @@ on:
     - cron: '0 4 * * *'        # 04:00 UTC, after the images (nightly.yml, 03:00)
   workflow_dispatch:
 
+concurrency:
+  group: qemu                     # every workflow shares this one group (Captain Bob, 2026-11-07: a
+  cancel-in-progress: false       # runner-image rebuild blocks other jobs, always): one run at a time
+
 jobs:
   docs:
     name: Book, volumes, API reference

.gitea/workflows/nightly.yml

@@ -20,7 +20,8 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: qemu                     # one QEMU at a time on the runner: shared with acceptance.yml
+  group: qemu                     # every workflow shares this one group: one run at a time,
+                                  # a runner-image rebuild included (Captain Bob, 2026-11-07)
   cancel-in-progress: false
 
 jobs:

.gitea/workflows/release.yml

@@ -14,7 +14,8 @@ on:
     tags: ['v*']
 
 concurrency:
-  group: qemu                     # one QEMU at a time on the runner: shared with acceptance.yml
+  group: qemu                     # every workflow shares this one group: one run at a time,
+                                  # a runner-image rebuild included (Captain Bob, 2026-11-07)
   cancel-in-progress: false
 
 jobs:

.gitea/workflows/runner-image.yml

@@ -17,6 +17,10 @@ on:
     paths: ['ci/runner-image/**', 'Makefile']
   workflow_dispatch:
 
+concurrency:
+  group: qemu                     # every workflow shares this one group (Captain Bob, 2026-11-07: a
+  cancel-in-progress: false       # runner-image rebuild blocks other jobs, always): one run at a time
+
 jobs:
   image:
     name: Build and publish the runner image