ci: set correct arch for rootfs tests

With the commit 01e5cfc "CI: Add target/arch tags (no suffix) for
snapshot images"[1] the os/platform is set for all images, which is usually
different from what the GitHub action runner uses (x86). The Docker
deamon still tries to fetch the x86 version and fails.

This commit explicitly sets the fitting arch.

[1]: openwrt/docker@01e5cfc

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit d359fa04eda29638b9326c194490685c1177fd49)
(cherry picked from commit da3b774)

.github/workflows/multi-arch-test-build.yml

@@ -147,11 +147,11 @@ jobs:
       - name: Build Docker container
         if: ${{ matrix.runtime_test && fromJSON(env.HAVE_IPKS) }}
         run: |
-          docker build -t test-container --build-arg ARCH .github/workflows/
+          docker build --platform linux/${{ matrix.arch }} -t test-container --build-arg ARCH .github/workflows/
         env:
           ARCH: ${{ matrix.arch }}-${{ env.BRANCH }}
 
       - name: Test via Docker container
         if: ${{ matrix.runtime_test && fromJSON(env.HAVE_IPKS) }}
         run: |
-          docker run --rm -v $GITHUB_WORKSPACE:/ci test-container
+          docker run --platform linux/${{ matrix.arch }} --rm -v $GITHUB_WORKSPACE:/ci test-container