Add BUILD_ARGs to ease use of proxy

[tibers]

Fixes #809 .

Description

Add BUILD_ARG which will reference either the environment or someone customizing the makefile. Can be used like BUILD_ARG = --build-arg https_proxy=http://myproxy:3128 --build-arg http_proxy=http://myproxy:3128 to improve the users proxy experience, for instance.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- Skaffold config changes like
  e.g. "Add buildArgs to `Kustomize` deployer skaffold config."
- Bug fixes
  e.g. "Improve skaffold init behaviour when tags are used in manifests"
- Any changes in skaffold behavior
  e.g. "Artiface cachine is turned on by default."

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[tibers]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[slmagus]

Ah ha! Thank you!

No worries, I got a PR in #810 that fixes it and make sure make images doesn't fail so hard in the future.