Install `jupyter-myst-build-proxy` to our docker image from conda-forge

[mfisher87]

See:

workshop-open-source-geospatial/docker/environment.yml

Lines 54 to 57 in 8030176

	# Pip-only dependencies... ideally we get these on conda-forge.
	- "pip"
	- pip:
	- "jupyter-myst-build-proxy" # Automatic build and proxying of MyST sites, triggered by requests to `$PREFIX/myst-build/$PATH_TO_MYST_PROJECT`

See: ryanlovett/jupyter-myst-build-proxy#13