ci: build from outside the container

This fixes the build by not running the action inside the container but using the Docker image in the action.

.github/workflows/build.yml

@@ -21,9 +21,13 @@ jobs:
       - name: Clone repository
         uses: actions/checkout@v2
         with:
+          path: project
           submodules: recursive
       - name: Build ZSWatch Binaries
-        run: west build --board ${{ matrix.board }} app
+        working-directory: project
+        run: |
+          docker run -v $PWD:/workdir/project nordicplayground/nrfconnect-sdk:v${{ matrix.ncs_version }}-branch \
+            west build --board zswatch_nrf5340_cpuapp@2 /workdir/project/app
 
   Posix:
     name: Posix build