ci: run on any runner, amd64 or arm64

The builder image is now multi-arch (linux/amd64 + linux/arm64). Both
variants were tested by running this repo's own CI build steps in each
before being pushed as :latest. Jobs therefore no longer need an amd64
host: runs-on moves to ubuntu-latest, which the amd64 runner (draco-x86)
and the ARM runner (oracle-a1) both carry, so jobs go to whichever is
free instead of queueing behind long jobs on draco.

The Dockerfile header now gives the buildx command for the multi-arch
image, so a rebuild does not silently go back to amd64-only.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -15,7 +15,7 @@ on:
 
 jobs:
   build:
-    runs-on: linux/amd64
+    runs-on: ubuntu-latest
     container:
       image: gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest
 

.gitea/workflows/release.yaml

@@ -13,7 +13,7 @@ on:
 
 jobs:
   build-and-release:
-    runs-on: linux/amd64
+    runs-on: ubuntu-latest
     container:
       image: gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest
 

.gitea/workflows/test.yaml

@@ -17,7 +17,7 @@ on:
 
 jobs:
   test:
-    runs-on: linux/amd64
+    runs-on: ubuntu-latest
     container:
       image: gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest
 

Dockerfile.builder

@@ -2,8 +2,11 @@
 # Pre-built image with the .NET 10 SDK and JPRM for building and testing the plugin. The SDK builds
 # both the net9.0 (Jellyfin 10.11) and net10.0 (Jellyfin 12) targets; the net9.0 test run rolls
 # forward onto the .NET 10 runtime.
-# Build: docker build -f Dockerfile.builder -t gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest .
-# Push:  docker push gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest
+# Multi-arch (amd64 + arm64): CI runs on ubuntu-latest, which both the amd64 and the
+# ARM runners carry, so the image must exist for both.
+# Build + push (needs a docker-container buildx builder and arm64 binfmt/QEMU):
+#   docker buildx build -f Dockerfile.builder --platform linux/amd64,linux/arm64 \
+#       -t gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest --push .
 
 FROM mcr.microsoft.com/dotnet/sdk:10.0