Create and publish docker image

[szkiba]

In certain circumstances, it can be useful to run k6x using the docker engine itself, as a drop-in replacement of the k6 docker image. Therefore, it is advisable to publish in the form of a docker image that contains the tools necessary for building (golang, git).

In addition, the k6x docker image can be used instead of the xk6 docker image for building, which allows for faster builds. The go cache (go build, go module) can be placed on a separate /cache volume, so it can easily be made persistent.