chore: fix docker image

[tjwelde]

No-Ticket

The integration tests on the SDK were failing, because the docker image for the standalone chain was panicking with following error:

Thread 'main' panicked at 'no CA certificates found',

Here is someone else having the same problem: paritytech/substrate#9984

I modified the Dockerfile to include the certificates. I was following substrates maintain Dockerfile here: https://github.com/paritytech/substrate/blob/7fbec2f9d22adc6bf968a048d768b2e30b46ac30/.maintain/docker/substrate.Dockerfile

It might not be necessary to switch to the ubuntu image. I did that, because I couldn't install anything in the docker image manually (while running it with /bin/bash) and substrate has switched to it: paritytech/substrate#9753. It turns out, that apt-get and similar are missing, because the whole usr binaries are removed: https://github.com/KILTprotocol/mashnet-node/blob/develop/Dockerfile#L34.

Do we really need to remove all these binaries?
Is it ok to switch to the ubuntu image?

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• This PR does not introduce new custom types
  • If not, I have opened a companion PR with the type changes in the KILT types-definitions repository
• I have left the code in a better state
• I have documented the changes (where applicable)