ci/cd: fix recent Docker build failures on macOS

The GitHub workflow for testing Docker builds on macOS was consistently
failing. This commit downgrades the macOS version used for Docker tests
to `macos-13`, which is the latest Intel-based macOS runner, instead of
the ARM-based `macos-14` which `macos-latest` points to.

This change is necessary because the hypervisor framework required for
Docker is not supported on the ARM-based macOS runners provided by
GitHub. This issue was causing failures when attempting to run Colima
with QEMU using `-accel hvf`, which is unsupported on these runners.
Switching to an Intel-based runner resolves this issue.

Related issues:
- actions/runner-images#9460
- actions/runner-images#9741
- abiosoft/colima#1023

.github/workflows/checks.build.yaml

@@ -72,16 +72,19 @@ jobs:
   build-docker:
     strategy:
       matrix:
-        os: [ macos, ubuntu ] # Windows runners do not support Linux containers
+        os:
+          - macos-13 # Later versions running on ARM fails due to lack of nested virtualization (actions/runner-images#9460, actions/runner-images#9741, abiosoft/colima#1023)
+          - ubuntu-latest
+          # - windows-latest # Windows runners do not support Linux containers
       fail-fast: false # Allows to see results from other combinations
-    runs-on: ${{ matrix.os }}-latest
+    runs-on: ${{ matrix.os }}
     steps:
       -
         name: Checkout
         uses: actions/checkout@v4
       -
         name: Install Docker on macOS
-        if: matrix.os == 'macos' # macOS runner is missing Docker
+        if: contains(matrix.os, 'macos') # macOS runner is missing Docker
         run: |-
           # Install Docker
           brew install docker