Override xstartup with environment variable

[manics]

Defines the variable JUPYTER_REMOTE_DESKTOP_PROXY_XSTARTUP to launch a custom xstartup script.

The main benefit of this is launching a different desktop e.g. https://github.com/manics/jupyter-desktop-mate/blob/main/start-mate.sh on a system where a user's home directory may be mounted, meaning ~/.vnc/xstartup doesn't exist.

Ideally this would be done with a traitlets configuration so it could be specified in one of the standard configuration paths, but the VNC server and desktop are launched using a custom jupyter-server-proxy entrypoint

jupyter-remote-desktop-proxy/setup.py

Lines 62 to 64 in 4f68d13

	'jupyter_serverproxy_servers': [
	'desktop-websockify = jupyter_remote_desktop_proxy.setup_websockify:setup_websockify',
	]

so as far as I know it's not possible to pass a traitlets configuration object to it.

[github-actions]

👈 Launch a binder notebook on this branch for commit 3a025bb

I will automatically update this comment whenever this PR is modified

[manics]

I've been prototyping a configurable entrypoint in jupyterhub/jupyter-server-proxy@58e5667 (jupyterhub/jupyter-server-proxy#509)

I'm undecided on what's better though, an env-var or the configurable.