Add support for additional builder arguments

[regiswilson]

This addresses my comments in #15

[72636c]

Heya, thanks for the PR! This looks sensible.

Thoughts on making this an array like the Docker Compose plugin?

[regiswilson]

Thoughts on making this an array like the Docker Compose plugin?

I certainly wish that all the plugins would be similar enough (if not identical). The ecr-cache plugin uses the name "additional build args" as a plain string which I think is slightly better than when they use the "args" name in the docker plugin. In truth, args/build_args/additional_build_args needs to be cleared up before we press on. But that's a nit among gnats!

1. I have no problem making it available as an array optionally (but with the problem that I would then want to also patch the ecr-cache plugin).
2. It appears there is a merge conflict at the moment with a "target" parameter (similarly aped from the ecr-cache and other plugins). Funnily enough, I had a branch already setup for a similar PR I was going to send you after this one. I had targeted in this PR v1.3.0 which was released very recently, so I need to merge-conflict and re-target 1.4.0.

So I propose (if you accept): We move ahead with this PR (post merge-conflict) while also opening issues in both this repo and the ecr-cache repo to make them both accept arrays and strings [as well as perhaps identify what the best course of naming for these parameters would be --optional].

[regiswilson]

This PR has been updated to avoid a conflict with v1.3.0 and retargets v1.4.0

[72636c]

I had another think about this one, let's just leave it as a string. Thanks again for the PR, I'll get this out after the target fix.