Change base image for xk6 image

[pablochacin]

Currently xk6 image is based on a Debian image, which brings a significant number of unneeded dependencies that increase the surface for vulnerabilities. Consider an image with a smaller attack surface such a alpine

However, this is potentially breaking change as:

1. Some users can use this image as a base for their own images and relay on some tools available on the debian for their build processes (e.g. git, curl) which are not available in the alpine
2. Images such as alpine doesn't have compatibility issues with extensions requiring cgo enabled (due to the lack of a proper standard glibc library)

[pablochacin]

Reopen after #87