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