Limit Docker tests to Linux

Author: s1hofmann

.github/workflows/snapshot.yaml

@@ -63,7 +63,7 @@ jobs:
         with:
           node-version: 14
       - name: Setup Docker
-        if: ${{matrix.os === 'ubuntu-latest'}}
+        if: ${{matrix.os == 'ubuntu-latest'}}
         run: |
           docker pull s1hofmann/nut-ci:latest
           docker run -it -d --name nut-ci --shm-size 4gb --user $(id -u):$(id -g) -v ${PWD}:${PWD}:rw s1hofmann/nut-ci:latest bash