Dockerfiles: Rename working directory from "/build" to "/work"

Since the directory will contain both sources and build, this commit
generalizes its name.

linux-arm64/Dockerfile

@@ -35,7 +35,7 @@ ENV AS=/usr/bin/${CROSS_TRIPLE}-as \
     LD=/usr/bin/${CROSS_TRIPLE}-ld
 
 ENV DEFAULT_DOCKCROSS_IMAGE dockcross/linux-arm64
-WORKDIR /build
+WORKDIR /work
 
 # Note: Toolchain file support is currently in debian Experimental according to:
 # https://wiki.debian.org/CrossToolchains#In_jessie_.28Debian_8.29