Try combined amd64/arm64 build

Split Dockerfile so z88dk doesn't have to be rebuilt every time it falls out od cache

.devcontainer/z88dk/Dockerfile

@@ -0,0 +1,55 @@
+# syntax=docker/dockerfile:1
+
+# Similar to the speccydev container, I'm building this with:
+# docker buildx build --platform linux/arm64/v8,linux/amd64 -t boarstone.mcphail.uk/mcphail/z88dk:tagname .
+
+FROM ubuntu:24.04 AS build
+RUN apt-get update \
+	&& apt-get install -y --no-install-recommends \
+	bison \
+	build-essential \
+	ca-certificates \
+	ccache \
+	cpanminus \
+	curl \
+	dos2unix \
+	flex \
+	gdb \
+	git \
+	libboost-all-dev \
+	libcapture-tiny-perl \
+	libclone-perl \
+	libdata-hexdump-perl \
+	libfile-slurp-perl \
+	libgmp3-dev \
+	liblocal-lib-perl \
+	libmodern-perl-perl \
+	libpath-tiny-perl \
+	libregexp-common-perl \
+	libtext-table-perl \
+	libxml2-dev \
+	libyaml-perl \
+	m4 \
+	make \
+	perl \
+	pkg-config \
+	ragel \
+	re2c \
+	texi2html \
+	texinfo \
+	zlib1g-dev \
+	&& rm -rf /var/lib/apt/lists/*
+ENV Z88DK_PATH="/opt/z88dk"
+RUN cpanm -l $HOME/perl5 --no-wget local::lib Template::Plugin::YAML \
+	&& git clone --depth 1 --branch v2.4 --recursive https://github.com/z88dk/z88dk.git ${Z88DK_PATH} \
+	&& cd ${Z88DK_PATH} \ 
+	&& eval "$(perl -I$HOME/perl5/lib/perl5 -Mlocal::lib)" \
+	&& chmod 777 build.sh \
+	&& BUILD_SDCC=1 BUILD_SDCC_HTTP=1 ./build.sh \
+	&& make install-clean bins-clean 
+
+FROM ubuntu:24.04
+COPY --from=build /opt/z88dk/ /opt/z88dk/
+ENV PATH="$(PATH):/opt/z88dk/bin"
+ENV ZCCCFG="/opt/z88dk/lib/config"
+USER ubuntu