Start preparing some automation helpers, update building docs a bit

[tylert]

The Dockerfile is intended to try to give folks another option for their local build environment. There is a chance this work will also aid in future attempts to get GitHub actions going for automating release building too.

The nrf52_post_prep.py helper Python script is intended to be called from the Makefile make prep-nrf target. However, I would appreciate some help testing this on non-Linux targets first before I call this on every build. It is needed because there isn't yet a new release of the Heltec_nRF52 dev env (HelTecAutomation/Heltec_nRF52#4).

[jacobeva]

Nice PR. Some notes:
Please create a scripts (capital S) folder and put the python script into it. This is just to keep things tidy.
The script should really be renamed. It's only needed when compiling for the T114 at the moment. heltec_nrf52_bsp_prep.py will do as the filename if you can't think of anything shorter. It needs to be clear what it's for else some may misunderstand.

Once this is done, I'm happy to merge. Thanks for the work Tyler.

[tylert]

Done.

We'll also have to wait for another pending change to the Dockerfile after there's a release to fix arduino/arduino-cli#2860 too.