Removed --on-server flag in build instruction

The --on-server option prevents proper building in the docker container and it's suggested not to be used.

--on-server is not a current option so maybe it’s an old fdroidserver that being said, on-server needs a VM via vagrant or libvirt, which one do you have? It appears more like you run this directly in a VM or on bare metal? If so then remove --on-server

By @licaon-kter in Build error: OSError while trying to execute sudo … No such file or directory: ‘sudo’

Can you run fdroid build --verbose com.gitlab.uak.myapp instead? Drop the on-server thing: fdroid build: hide --on-server cli flag (!617 (merged)) · Merge requests · F-Droid / fdroidserver · GitLab

By @licaon-kter in Submitting a Kivy App build errors

--on-server should not part of fdroids public API. Considering what that flag does it's clearly not suitable for any use-case other that running fdroid build inside our automated builder VM. Especially some security measures --on-server deploys, will certainly cripple the operating systems of users.

by @mimi89999 in fdroidserver!617 (merged)