build: test container: don't pull from docker hub; half support apk files

[Habbie]

as requested by @aparcar, this fetches rootfs files from downloads, instead of relying on what's on docker hub.

TODO before merge:

• disable, or fix, mips_24kc

TODO later (hopefully somebody else can pick this up after this is merged):

• test .apk files in entrypoint.sh

master test run

24.10 test run

[Habbie]

the test runs used a slightly different version, because I had to get the .py script from my fork. Here's the diff:

diff --git a/.github/workflows/multi-arch-test-build.yml b/.github/workflows/multi-arch-test-build.yml
index 685abc5..cd1614c 100644
--- a/.github/workflows/multi-arch-test-build.yml
+++ b/.github/workflows/multi-arch-test-build.yml
@@ -66,12 +66,6 @@ jobs:
           echo "Building for $BRANCH"
           echo "BRANCH=$BRANCH" >> $GITHUB_ENV
 
-# we have matrix.arch from above
-# we have BRANCH=master or openwrt-23.05 or openwrt-24.10 etc.
-# for 23.05, we want something from https://downloads.openwrt.org/releases/23.05-SNAPSHOT/targets/
-# for master, https://downloads.openwrt.org/snapshots/targets/
-# for, say, powerpc_8548, inside targets/, we want mpc85xx/p1010 (so from matrix.target)
-
       - name: Determine changed packages
         run: |
           # only detect packages with changes
@@ -208,8 +202,7 @@ jobs:
         if: ${{ matrix.runtime_test && fromJSON(env.HAVE_PKGS) }}
         uses: actions/checkout@v4
         with:
-          repository: habbie/openwrt-actions-shared-workflows
-          ref: docker-import
+          repository: openwrt/actions-shared-workflows
           path: dockerfiles_feeds
           sparse-checkout: |
             .github/scripts/ci_helpers.sh
@@ -218,7 +211,7 @@ jobs:
             .github/dockerfiles_feeds/entrypoint.sh
           sparse-checkout-cone-mode: false
 
-      - name: Get rootfs file name
+      - name: Get rootfs
         if: ${{ matrix.runtime_test }}
         run: |
           ROOTFSFILE=$(dockerfiles_feeds/.github/scripts/get-rootfs-url.py)

(the comment is gone because that also lives in .py now)