cmd/gitserver: add p4-fusion installation to gitserver Docker image
Created by: sashaostrikov
I ended up installing p4-fusion directly in resulting image, because using latest alpine and git and building p4-fusion
as a stage and copying the files to resulting container seem to lose some files which are needed for successful p4-fusion
launch.
Container size increased from 570 MB to 688 MB
Closes https://github.com/sourcegraph/sourcegraph/issues/26926