Add GzipFastest & use it for the context

This PR adds a GzipFastest compression type which is used by CompressStream to set the gzip compression to level 1.
This PR makes `docker build` compress the context using GzipFastest to reduce the size of the uploaded context without affecting performance.

This lowers the amount of data `make build` has to upload from ~48 MB down to ~29 MB. This should give a small speed boost for small contexts and a bigger one to large contexts.