Support building on an arm64 host too

Dockerfile.build

@@ -3,6 +3,7 @@ FROM debian:bookworm-slim
 
 RUN set -eux; \
 	apt-get update; \
+	dpkgArch="$(dpkg --print-architecture)"; \
 	apt-get install -y --no-install-recommends \
 		ca-certificates \
 		gnupg \
@@ -12,6 +13,8 @@ RUN set -eux; \
 		libc6-dev \
 		make \
 		\
+# these are all "arch:all" so we can just install all of them
+		libc6-dev-amd64-cross \
 		libc6-dev-arm64-cross \
 		libc6-dev-armel-cross \
 		libc6-dev-armhf-cross \
@@ -21,7 +24,9 @@ RUN set -eux; \
 		libc6-dev-riscv64-cross \
 		libc6-dev-s390x-cross \
 		\
-		gcc-aarch64-linux-gnu \
+# the cross-compilers are particular about which architectures they build for, so for now we'll only support a host architecture of amd64 or arm64
+		$([ "$dpkgArch" = 'amd64' ] || echo 'gcc-x86-64-linux-gnu') \
+		$([ "$dpkgArch" = 'arm64' ] || echo 'gcc-aarch64-linux-gnu') \
 		gcc-arm-linux-gnueabi \
 		gcc-arm-linux-gnueabihf \
 		gcc-i686-linux-gnu \