Add support for --build-arg

[FelixDuvalletKodiak]

Some Dockerfiles contain arguments:

ARG SOME_ARG="default_value"

and are filled in during building: docker build --build-arg SOME_ARG=my_value

Can we add support for a list of build arguments in this orb?

[nihonjinrxs]

Was looking at this for a possible Hacktoberfest contribution, but it seems like maybe #8 solved it? Is that not true?

[lokst]

@nihonjinrxs You're right, the work done in #8 can be used to resolve this issue. Apologies for the confusion, and thanks for reaching out to clarify!