Configures Enterprise Edition image to automatically start with enterprise features enabled

[Spoffy]

Updates the Github workflow for docker_latest to separate out the Grist EE image build, as it requires special settings.

Additionally, parameterises a number of settings to make forking and debugging easier.

[berhalak]

Is the title of this PR accurate and aligned with the description?

[paulfitz]

Is the title of this PR accurate and aligned with the description?

Yes, it is to fix something in the grist-ee image that broke recently. Perhaps the word "enabled" is ambiguous. It means that the grist-ee image when started will act as grist-ee has always acted (until it broke recently), and use material in grist-ee/ext. When it broke, it stopped doing that until an activation key was provided in the environment, so trialing stopped working. The most important line in the change is file: ext/Dockerfile which is used to set an environment variable in the image.

[paulfitz]

Thanks @Spoffy