Decentralized Devstack changes: Add and push Dockerfile; add decentralized devstack settings (#24666)

* Add and push Dockerfile; add decentralized devstack settings

Co-Authored-By: Diana Huang <dkh@edx.org>
Co-Authored-By: Kyle McCormick <kmccormick@edx.org>

* Remove Python requirements hack

Remove the attempted optimization to the installation of Python
package dependencies.  The dependencies in edx-platform change
about three times per day, so this was of dubious value.  And
because npm is run through nodeenv, which is a Python package,
the Python dependencies installation has to happen first.

* ARCHBOM-1439: Changing workdir to /edx/app/edxapp/edx-platform (#24835)

Context: The Dockerfile tries to stay in sych with legacy stuff.
In the ansible we configure the directory structure such that things
relating to the app but not in the codebase,
such as the env file wind up in /edx/app/edxapp/.
And the codebase winds up in /edx/app/edxapp/edx-platform.

I think due to accident, the...