DOCS: Add section how to install a few packages on top of notebook images

[jbusecke]

Is your feature request related to a problem? Please describe.
Over at LEAP I am working with lots of researchers who want to install packages on top of one of the pangeo images. Our current recommendation is to build a custom docker image but I am not entirely sure how to encode this most cleanly in a Dockerfile.

Describe the solution you'd like
The docs currently contains instructions on how to build a fully custom image from base-image. It would be great if we could add another section on how to install a single/few packages on top of e.g. pangeo-notebook.

It could be as simple as providing an example Dockerfile similar to this (I actually am not sure what the right way to do this is... so this might be pseudo-ish code):

# Inherit from an upstream image
FROM pangeo/pangeo-notebook:2023.11.14

# Option 1: Provide a list of packages
RUN mamba install package_a package_b
RUN pip install package_c
# NOTE: This does not work for me currently and I guess I need to activate the conda env somehow?

# Option 2: Provide either/or environment.yml/requirements.tx
COPY environment.yml /tmp/environment.yml
COPY requirements.txt /tmp/requirements.txt

RUN mamba env update ??? --file /tmp/environment.yml
RUN pip install -r /tmp/requirements.txt

Note the above is not properly working and I suspect that I am missing the conda env (notebook?) activation step. Any help to do this properly would be greatly appreciated.

cc @yuvipanda

[weiji14]

There's some docs at https://pangeo-docker-images.readthedocs.io/en/latest/howto/custom-image.html (added in #473) on how to build a custom image, mainly pointing to https://github.com/pangeo-data/pangeo-binder-template. Probably still good to mention how to configure such a Dockerfile though, if someone wants to add them to this file, go ahead!

pangeo-docker-images/docs/howto/custom-image.md

Lines 1 to 3 in aa43825

	# Build a custom image
	Advanced users may want a highly customized environment that still works on Pangeo BinderHubs. You can do that by building off the pangeo `base-image` following our [template repository example](https://github.com/pangeo-data/pangeo-binder-template). Further documentation on the configuration files in the `binder` subfolder can be found in the [repo2docker documentation](https://repo2docker.readthedocs.io/en/latest/config_files.html#configuration-files).