Cannot run Google App Engine custom managed VM: --custom-entrypoint must be set error

Question

PROBLEM DESCRIPTION

I am trying to create a custom managed VM for Google App Engine that behaves identically to the standard python27 managed VM pro

Answer 1

UPDATE

THIS MAY NO LONGER BE ACCURATE. SEE NICK'S ANSWER.

(Though I could not get that working. But I did not try very hard)

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There is a completely undocumented but absolutely essential piece of information w.r.t. custom managed VMs:

THEY CANNOT BE RUN ON THE DEVELOPMENT SERVER!

If you think this crucial fact would be mentioned anywhere sane, like say, the documentation page for custom managed VMs, or for dev_appserver.py, or even as an error message when running dev_appserver.py, then you are giving Google far too much credit.

The only place where I can find any kind of statement about this is in the Readme file of the appengine-java-vm-guestbook-extras demo on github (seriously):

The Cloud SDK does not support anymore running custom runtimes when a Dockerfile is provided. You'll have to deploy the application to App Engine

Google does not care to:

1. Implement this basic and important feature.
2. Document that the development server is missing such an important feature.
3. Give any reasonable error message when the user tires to perform the action.

I hope this answer saves some sorry developer from the days of torment I suffered because of this.