Kotlin support

[rvolosatovs]

Summary

Add Kotlin support. Refs https://github.com/TheThingsNetwork/api/issues/43

How do you propose to implement this?

Investigate https://github.com/streem/pbandk

Can you do this yourself and submit a Pull Request?

yes

[rvolosatovs]

@jpmeijers perhaps you would be interested in contributing to this? You can see some examples:

• https://github.com/TheThingsIndustries/docker-protobuf/pull/64
• https://github.com/TheThingsIndustries/docker-protobuf/pull/49

[strophy]

I researched this for a while and got the build and resulting binary working fine, but I got stuck running it because our target container runs as root, and the way the Kotlin file is made executable using Spring Boot means it will always output a warning if it runs as root: https://github.com/streem/pbandk/issues/73

@rvolosatovs are you familiar with gosu or su-exec? Do you think it makes sense to change our target image to use an unprivileged user to run protoc? We could probably create a user in an entrypoint and set it to the same permissions as the mounted workdir like this? We need to avoid making it a breaking change when doing this, for users that don't set a workdir...