[build] add --platform to Dockerfiles

.github/actions/muslbuilder/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.16
+FROM --platform=linux/amd64 alpine:3.16
 
 LABEL com.github.actions.name="C++ MUSL Builder Slim"
 LABEL com.github.actions.description="Provides a C++ MUSL environment"

demo/Dockerfile

@@ -1,5 +1,5 @@
 
-FROM debian:11.3-slim
+FROM --platform=linux/amd64 debian:11.3-slim
 
 RUN set -eux; \
 	export DEBIAN_FRONTEND=noninteractive; \