Tweak to cross-build aarch64-unknown-linux-musl on macOS M2

Author: tanishiking

README.md

@@ -1,3 +1,9 @@
+```sh
+$ TARGET=aarch64-unknown-linux-musl make build-wasmtime
+```
+
+---
+
 <div align="center">
   <picture>
     <source media="(prefers-color-scheme: dark)" srcset="./art/logo/runwasi_icon3.svg">

cross/Dockerfile.musl

@@ -1,8 +1,15 @@
-ARG CROSS_BASE_IMAGE
-FROM $CROSS_BASE_IMAGE
+# ARG CROSS_BASE_IMAGE
+# FROM $CROSS_BASE_IMAGE
+FROM ghcr.io/cross-rs/aarch64-unknown-linux-musl:0.2.5
 
 COPY --from=jorgeprendes420/apk-anywhere / /
-ENV MARCH=${CROSS_CMAKE_SYSTEM_PROCESSOR}
+
+# not inherited (why?)
+# https://github.com/cross-rs/cross/blob/main/docker/Dockerfile.aarch64-unknown-linux-musl
+# ENV MARCH=${CROSS_CMAKE_SYSTEM_PROCESSOR}
+ENV MARCH=aarch64
+ENV CROSS_SYSROOT=/usr/local/aarch64-linux-musl
+
 RUN apk-init ${MARCH} ${CROSS_SYSROOT}
 
 # configure libsecccomp-rs to use static linking