Docker build is not using cache
From your Dockerfile, if you append lines to your Dockerfile, or change the code being built, there's only one line that could be cached:
RUN mkdir /codeAfter that, you perform a:
COPY . /codeSince you have changed your Dockerfile, the contents of . have changed (the Dockerfile is part of .) and therefore the COPY needs to be performed again, generating a new layer. Once you generate a new layer, every following layer no longer has a cache and needs to be rebuild.
To improve caching, consider placing the lines that change less towards the top of your Dockerfile. That would leave the COPY . /code line at the very end of the file since it will change almost every time.