Add support for specifying platform and target arch in the manager image

[UnstoppableMango]

What this PR does / why we need it:

Adds support for building ARM manager images

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

Doesn't fix, but works towards implementing #251

Additional information

I couldn't find exactly how images were built and published, so I haven't made any workflow changes.

Similarly, I wasn't sure if there were any good spots to add tests for this. If any tests need to/should be added please let me know!

Special notes for your reviewer

I am building the image manually in another project of mine and it's running fine in my raspberry pi management cluster at the moment. Here is the dockerfile and a built image

[vmwclabot]

@UnstoppableMango, you must sign our contributor license agreement before your changes are merged. Click here to sign the agreement. If you are a VMware employee, read this for further instruction.

[vmwclabot]

@UnstoppableMango, we have received your signed contributor license agreement. The review is usually completed within a week, but may take longer under certain circumstances. Another comment will be added to the pull request to notify you when the merge can proceed.

[vmwclabot]

@UnstoppableMango, VMware has approved your signed contributor license agreement.