Fixing MAX_JOBS in Dockerfile, resolving issue #91

Hej, after running into OOM, I fixed the MAX_JOBS issue #91. The only file that needed to be modified is the Dockerfile. _Explanation_ Build arguments (ARG) are only visible at Dockerfile level, they're not passed through into the container. Therefor MAX_JOBS needs to be passed to the container as environment variable which only can be done bei ENV. That, on the other hand, cannot be overriden by a command line option like "--build-arg" (there's no "--build-env"). To be configurable, it needs to be defined as ARG first and then set with ENV while refering to the ARG: ``` ARG MAX_JOBS ENV MAX_JOBS=$MAX_JOBS RUN pip3 install ... ``` _Med hilsen fra Danmark_