Update .maintain/Dockerfile

[TriplEight]

A follow-up from #9753

is it still used?

Yes, and it is critical to keep it.

if yes, it should follow our containers policy and use ubuntu:20.04 as base

I see no issue with that. Do you want to include that in the PR? If not, please open an issue and I will take care of it.

and have some metadata as the other images do

100%

Originally posted by @chevdor in #9753 (comment)

[chevdor]

• add metadata
• use ubuntu:20.04 as base
• add label io.parity.image.type="builder"
• remove labels not relevant to images not built via CI
• align the entrypoint with the injected image
• align the volumes with the injected image
• fix exposed ports

[chevdor]

See also paritytech/polkadot#3884