rootfs container builds failing due to recent targz changes upstream

[efahl]

@aparcar @7Ji

One of our recent changes in the image.mk upstream (https://github.com/openwrt/openwrt/commits/main/include/image.mk) is now generating two files that match the wildcard at https://github.com/openwrt/docker/blob/main/.github/workflows/containers.yml#L474, resulting in failures like this https://github.com/openwrt/docker/actions/runs/26712024650/job/78729065244#step:9:381

https://mirror-03.infra.openwrt.org/snapshots/targets/x86/64/ now has both of these that are matching. The first one is "new", i.e., it only appears since the image.mk file changes.

openwrt-x86-64-generic-targz-rootfs.tar.gz
openwrt-x86-64-rootfs.tar.gz

[efahl]

@aparcar @ynezz

How does this look? I have no way of testing, so I'm reluctant to PR it without some eyes on it first...

index 5e2efa6adee8..e0bc96208fd6 100644
--- a/.github/workflows/containers.yml
+++ b/.github/workflows/containers.yml
@@ -471,11 +471,12 @@ jobs:
         id: build_args
         run: |
           echo 'args<<EOF
-            DOWNLOAD_FILE=openwrt-.*-rootfs.tar.gz
+            TARGET=${{ matrix.target }}
+            TARGET_TAG="${TARGET//\//-}"
+            DOWNLOAD_FILE=openwrt-${TARGET_TAG}-rootfs.tar.gz
             WORKDIR=/
             USER=root
             VERSION_PATH=${{ needs.generate_matrix.outputs.version_path }}
-            TARGET=${{ matrix.target }}
             BASE_IMAGE=scratch
             CMD=ash
             FILE_HOST=${{ needs.generate_matrix.outputs.file_host }}