Add demo target to Dockerfile

[mathemancer]

Fixes #3522

This adds a build stage to the Dockerfile for version 0.1.6, and adds a setting to fix the issue #3497 , but in the demo setting.

Edit: Now the default build is production, other targets work as expected.

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[mathemancer]

@pavish I converted this to draft, since we may want to change the approach.

[mathemancer]

It would violate DRY, but couldn't we just base prod and demo off of the base in the same way, copy the build from develop into both, etc.? Then if production was last in the file, it would be the one produced by default (I think).

[mathemancer]

Okay, I reordered things so production should be the default. I'm going to double-check that everything works as expected, and then mark this ready for review.

[pavish]

@mathemancer Looks good to me!