Regula server docker image is still stuck at v0.5.0

[tealeg]

This issue has mutliple parts:

1. the docker image in the repository is still set to buld v0.5.0, and for me doesn't even build with a current version of docker.
2. the ./.ci/docker.sh script that is run by travis on this repository is failing every time it runs, so we're not pushing any new builds to quay.io. The last succesful push is a year old!

... this is causing issues in development because aww pulls in the docker image and people are testing rules against v0.5.0 instead of v0.6.1, which means valid rules including the Percentile and FNV expressions are being rejected.

[tealeg]

Context:
@sixstone-qq was testing a new rule in his local environment and got an "unknown expression kind" error back, that he shouldn't have seen. That's because he was running the docker image as a server (v0.5.0) but using v0.6.1 as the client code. The result of this is that he can't test his new rule locally and it makes it hard for him to have confidence about putting it into production.