Put Dockerfile.netkan in build context

[HebaruSan]

Problem

#2838 added a build process to rebuild and push a Docker inflator container, but it failed:

https://travis-ci.org/KSP-CKAN/CKAN/jobs/569174846#L4638-L4642

========================================
docker-inflator
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unable to prepare context: the Dockerfile (/home/travis/build/KSP-CKAN/CKAN/Dockerfile.netkan) must be within the build context
An error occurred when executing task 'docker-inflator'.

Cause

Prior to docker/cli#886, the Dockerfile specified by the -f parameter had to be in the build directory for... some reason.

That requirement was removed in Docker 18.03.0-ce. Travis has 17.09.0-ce. I have 18.09.7. So when I tested #2839 locally, it worked fine, then failed when pushed to master.

Changes

Now we copy Dockerfile.netkan into _build/docker/inflator before building.
If Travis ever upgrades to Docker 18.03.0-ce or later, we can revert this.

[techman83]

Thanks @HebaruSan LGTM! I haven't fully dug into build contexts, but likely it was picked up within the context automatically before the ability to specify a filename existed. And why being aware of what ends up in the context is important -> https://medium.com/lucjuggery/docker-tips-about-the-build-context-dbc76505e178