Make LocalStack image configurable

[aosagie]

Hi,
In order to use Test Containers and LocalStack at my company, we need to resolve images from our corporate registry. Currently, LocalStackContainer's image is hard coded. This PR makes it configurable. I followed the lead of a prior commit that made KafkaContainer's image configurable: 4e15af82

I noticed a prior PR attempted to do this: #1604
But

1. Its development seems to have been stalled for the past couple months
2. It's slightly more complex than necessary

Thanks for your time!

[rnorth]

Thanks for contributing @aosagie!
I think this looks fine really. It obviously lacks a test, but it's clear enough to see its behaviour and consistent with the other image name properties.

[aosagie]

Thanks for the prompt review @rnorth
Is there anything additional I need to do to get this merged or is everything good to go?

[rnorth]

@aosagie sorry no, nothing needed - wanted to give other reviewers a chance to look before merging, but will merge myself now.

[rnorth]

Merged. Thank you for the contribution @aosagie!