Allow authors to install custom python packages

[mpadge]

@csaybar @maurolepore I'll update the Dockerfile for our pkgcheck system with a space to allow package authors to add custom python dependencies required for their package to pass checks. @csaybar I'll add earthengine-api, so you won't have to do anything here.

[mpadge]

@csaybar That commit installs earthengine-api to our build system, but as a global install rather than part of any python environment. Your package still won't install properly, because reticulate only works via environments. Can you please help by submitting a PR to setup the required python environment for your package. The format should follow the final Rscript lines in the current Dockerfile:

pkgcheck/Dockerfile

Lines 257 to 261 in 48e7ec5

	RUN Rscript -e 'arrow::install_arrow()'
	# Plus current ubuntu-unstable versions cause failed linkage of sf to GEOS, so
	# need to reinstall 'sf' without bspm:
	RUN Rscript -e 'bspm::disable();install.packages("sf");bspm::enable()'

Please add directly below the lines inserted in the above commit (pip install). Thanks! (And of course, this still won't really solve your specific issues, because earthengine-api requires authorization, but the general procedure established here will still help others.)