docs: Update getting started dockerfile

[rach-id]

Ubuntu 16.04 still installs Python:2.7 and pip. Thus, the getting-started docker image doesn't build as expected. So, an update to Ubuntu:20.04 is needed for the images to build successfully.

[dlz21]

This fixes build error, how can we get someone to look at this?

[rach-id]

If someone is interested in merging this, I can fix the pipelines :D

[WadeBarnes]

@SweeXordious, You need to add DCO sign-off to your commits. Details here; https://github.com/hyperledger/indy-sdk/pull/2438/checks?check_run_id=4056269767

[rach-id]

@WadeBarnes Can you please let me know why the CI is not passing ? I don't have access to jenkins

[WadeBarnes]

@WadeBarnes Can you please let me know why the CI is not passing ? I don't have access to jenkins

It's a rust (cargo) version issue that is failing the builds. https://github.com/hyperledger/indy-sdk/pull/2452#issuecomment-988177968

More specifically; https://github.com/hyperledger/indy-sdk/issues/2460

[rach-id]

Thanks, will wait until it's fixed then rebase

[WadeBarnes]

A fix for the build issues has been implemented (https://github.com/hyperledger/indy-sdk/pull/2470). Please rebase this PR on the latest from the main branch so it is no longer blocked by the related build issues.

[WadeBarnes]

@SweeXordious, Sorry, it looks like the merge from another PR has created a conflict with your change. Could you please review.

[rach-id]

@WadeBarnes Thanks for letting me know. Conflicts resolved.