Pre-compress assets

#### Is your suggestion related to a problem? Please describe.
The assets (most notably JS) are served uncompressed when running from the Docker image, which becomes problematic on bandwidth-limited connections.

#### Describe the solution you'd like.
That the assets be pre-compressed.

#### Describe alternatives you've considered.
N/A

#### Additional context
Some users might want override certain files, such as config.json. See #16837

[zopfli](https://github.com/google/zopfli) was suggested as a gzip alternative by @Glandos

See also #16698.