update.sh: extract local tarball (multiarch#66)

Rules `ADD` follows:
- If `<src>` is a URL and `<dest>` does not end with a trailing slash, then a file is downloaded from the URL and copied to `<dest>`.

- If `<src>` is a URL and `<dest>` does end with a trailing slash, then the filename is inferred from the URL and the file is downloaded to `<dest>/<filename>`. For instance, `ADD http://example.com/foobar /` would create the file `/foobar`. The URL must have a nontrivial path so that an appropriate filename can be discovered in this case (http://example.com will not work).

Author: tcely

update.sh

@@ -28,11 +28,16 @@ to_archs="aarch64 aarch64_be alpha armeb arm cris hppa i386 m68k microblazeel mi
 
 for to_arch in $to_archs; do
     if [ "$from_arch" != "$to_arch" ]; then
-        docker build -t ${REPO}:$from_arch-$to_arch -<<EOF
+        mkdir -p ${from_arch}_qemu-${to_arch}
+        curl -sSL -o "${from_arch}_qemu-${to_arch}/${from_arch}_qemu-${to_arch}-static.tar.gz" \
+            "https://github.com/${REPO}/releases/download/v${VERSION}/${from_arch}_qemu-${to_arch}-static.tar.gz"
+        cat > ${from_arch}_qemu-${to_arch}/Dockerfile -<<EOF
 FROM scratch
-ADD https://github.com/${REPO}/releases/download/v${VERSION}/${from_arch}_qemu-${to_arch}-static.tar.gz /usr/bin
+ADD ${from_arch}_qemu-${to_arch}-static.tar.gz /usr/bin/
 EOF
+        docker build -t ${REPO}:$from_arch-$to_arch ${from_arch}_qemu-${to_arch}
         docker tag ${REPO}:$from_arch-$to_arch ${REPO}:$to_arch
+        rm -rf ${from_arch}_qemu-${to_arch}
     fi
 done