Travis CI demands payment

[Yilin-Yang]

Travis CI is no longer automatically triggering new builds because I've run out of build credits.

I still want a way to easily test code changes against multiple different (neo)vim versions. Am writing a Dockerfile to loosely replicate the Travis build and testing environment that I can use for offline testing. If I decide to buy build credits again, I might be able to publish and reuse that Dockerfile.

[Yilin-Yang]

Managed to set up a Docker image that aggregates binaries for vim v8.1.0039 and neovim v0.3.4 so that developers can (in principle) perform a thorough Travis CI-esque test run on their machines.

However, tests that pass in my own regular development environment fail when run from inside the Docker container, which renders it useless as a dev tool. In particular, the l:marker (a Unicode right-facing arrow) used to highlight the cursor position in the markbar context would inexplicably revert to a null string when being used in MarkbarTextGenerator. These issues only seem to happen inside the container.

Other failures include E484: Can't open file /root/out/vim8.1.0039/bin/vim/syntax/syntax.vim when trying to run the Docker image's (n)vim binaries.