refactor: move OCI image.source LABEL out of Dockerfile

Defining the LABEL via an ARG at the top of the Dockerfile prevents other GitHub repositories (or local Docker users) from leveraging otherwise identical cache layers.

PR: #18

Dockerfile

@@ -4,9 +4,6 @@ CMD ["/bin/bash"]
 
 ENV DEBIAN_FRONTEND=noninteractive
 
-ARG REPOSITORY_URL=https://github.com/innovaker/zmk-docker
-LABEL org.opencontainers.image.source ${REPOSITORY_URL}
-
 ARG ZEPHYR_VERSION
 ENV ZEPHYR_VERSION=${ZEPHYR_VERSION}
 RUN \