This `web.HttpFileHandler(srv, "/provider.json", opts.ApiPath+"provider.json")`
should have been `opts.ApiPath+"/provider.json"`. The difference is there was a
missing / before provider.json. That results in vpnweb trying to open a path
that doesn't exist: /etc/leap/config/vpnprovider.json.

Because this is so hard to see, its better to use filepath.Join, so these things
cannot happen. So I've switched these to the standard library function that
joins paths.

and add unit tests for message parsing

- Resolves: #5

- Resolves: #6

Add a Dockerfile and gitlab-ci

See merge request !3

Upstream api change

See merge request !1

Made to be easy to run from float

Shapeshifter-dispatcher refactored how to deal with the arguments,
they want iatMode now not iat-mode.