Add debuggable steps

[Eriner]

Clear and concise description of the problem

Sometimes builds exhibit problems in CI that are not reproducible on a developer machine. Often this has to do with networking, environment, or other oversights. Often, if at a shell, a developer could easily solve these problems. Instead, they must push many "printfs".

Suggested solution

Add the ability for developers to connect to a shell via ssh at a step.

Alternative

Status quo (same with GitHub Actions, Drone, and others), developers must "printf" or painfully modify their build to minimize build times between steps for debugging speed.

Additional context

similar: #244

Validations

• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[6543]

I think we should solve this in a generic way:
would require to add a new function "attach" to pipeline backend interface

and implement it ...

[6543]

questioins to solve:

• how do you specify a step to be debugged
• at whitch point you should be able to attach
• make sure steps are not closed right away ... if?

[johanvdw]

Drone allows starting a tmate debug session.
https://community.harness.io/t/feature-preview-debug-mode/11474/1
Also for github actions such a plugin exists:
https://github.com/marketplace/actions/debugging-with-tmate-and-zsh

I do think restarting a pipeline with a tmate terminal in a specific place may be a nice solution.

[wailashi]

With Concourse CI you can open a shell in any currently or recently running build container using their command line tool.
Especially nice that it doesn't require any changes to the pipeline.