feat: add the ability to set specific images via ZeebeClusterBuilder

[aivinog1]

Description

I've added the convenience methods to set a docker image for the Zeebe container in the cluster.

Related issues

closes #139

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[aivinog1]

Hey @npepinpe! I see that you are the main contributor to this project, can you start workflows? :)

[npepinpe]

Hi Alexey, thanks for the PR! This seems like a reasonable feature - it's a little cumbersome having to iterate over brokers/gateways after building the cluster to set these properties. I think we may want to take a different approach to customizing them eventually - but as this is an experimental API, there's no reason for us not to play around with different approaches.

One small comment on the tests: I found that describing the assertions help a lot with readability, especially for others. You'll notice almost all assertions have an as method which gives more context. It's important when writing tests not only to ensure they're correct, but also that when they fail, we provide as much context as possible so the person looking at it (who may not have written the test initially) knows what needs to be fixed.

[aivinog1]

Hey @npepinpe. Can you take another look?

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[npepinpe]

Looks good!