Update: use less layers in Docker image

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Relates to #459

With this PR, I'm trying to initiate some move/improvements with the Docker image structure. The image uses a lot of layers for no obvious reasons. Let's try reduce the number of layers for size optimization and update speed.

Unfortunately, the original image doesn't build properly:

#16 4.759 E: Version '85.0.4183.102-1' for 'google-chrome-stable' was not found

If one of the maintainers could look into this or provide some information about this error, that would be great.