Mount the source volume rather than copy it

Now that the build script cleans up both before and after building, this
is a viable option and will significant reduce build times by promoting
container reuse (with `-k`).

deployment/Dockerfile.debian.arm64

@@ -32,11 +32,8 @@ RUN dpkg --add-architecture arm64 \
 # Link to build script
 RUN ln -sf ${SOURCE_DIR}/deployment/build.debian.arm64 /build.sh
 
-# Create the source dir
-RUN mkdir -p ${SOURCE_DIR}
+VOLUME ${SOURCE_DIR}/
 
 VOLUME ${ARTIFACT_DIR}/
 
-COPY . ${SOURCE_DIR}/
-
 ENTRYPOINT ["/build.sh"]