dockerfiles: upgrade to ubuntu:20.04

[TriplEight]

Polkadot companion: paritytech/polkadot#3828

I'm upgrading the CI image from being based on debian:buster to ubuntu:20.04, some details: paritytech/scripts#309
The binaries produced in our new CI image will be linked by the newer GLIBC so won't be executable in debian:

/usr/local/bin/adder-collator: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.29' not found (required by /usr/local/bin/adder-collator)
/usr/local/bin/adder-collator: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by /usr/local/bin/adder-collator)

@chevdor I wanted to ask you about https://github.com/paritytech/substrate/blob/master/.maintain/Dockerfile

• is it still used?
• if yes, it should follow our containers policy and use ubuntu:20.04 as base
• and have some metadata as the other images do

[chevdor]

LGTM

[chevdor]

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

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