Added environment variable (with default) to customize the container base image

[bgolinvaux]

Added new base_image input, defaults to alpine:3.19

[ben-z]

Thanks for the PR! Does this work? Build args appear unsupported by GitHub: gruntwork-io/terragrunt-action#74 (comment)

[bgolinvaux]

I am truly sorry! (and terribly stupid). I have this habit of creating the PR right away when I start working, but I meant to create it in my repo, just to track the branch and then create another one in the upstream and, instead, I opened the PR in your repo and forgot to set it to draft.

Indeed, it does not work and I considered manually calling docker, but it's a PITA. I'll close it for now

Thanks for the mutex, btw, it's super handy!