Add ARM builds

Now that https://github.com/mumble-voip/mumble/pull/5960 has been
merged, building on ARM should no longer suffer from failing test cases
and thus the corresponding images can now be enabled here.

See also #3

.github/workflows/ci.yml

@@ -28,4 +28,4 @@ jobs:
             docker_version: '0'
             publish: false
             update_latest: true
-            platforms: "linux/amd64"
+            platforms: "linux/amd64,linux/arm64"

.github/workflows/manual_dispatch.yml

@@ -28,5 +28,5 @@ jobs:
             docker_version: ${{ github.event.inputs.docker_version }}
             publish: ${{ github.event.inputs.publish == 'true' }}
             update_latest: ${{ github.event.inputs.update_latest == 'true' }}
-            platforms: "linux/amd64"
+            platforms: "linux/amd64,linux/arm64"
         secrets: inherit

.github/workflows/upstream_release.yml

@@ -12,5 +12,5 @@ jobs:
             docker_version: '0'
             publish: true
             update_latest: ${{ github.event.client_payload.is_latest }}
-            platforms: "linux/amd64"
+            platforms: "linux/amd64,linux/arm64"
         secrets: inherit