fix: armv7 docker builds

[groggemans]

Moves the ARCH build argument after the first FROM so it has the proper
value.
(https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact)

Adds the GOARM=7 option to the armv7 GO build.

What type of PR is this?
/kind bug

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #239

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

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[andyzhangx]

/ok-to-test

[groggemans]

I pushed a commit that fixed some references in release-tools/build.make but it seems that's not allowed in this repo. So I'll leave away the default value in the docker file so it retains the old behavior but the explicit overwrite does work.

[coveralls]

Pull Request Test Coverage Report for Build 1407858082

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.466%

---

Totals
Change from base Build 1384082910:	0.0%
Covered Lines:	532
Relevant Lines:	678

---

💛 - Coveralls

[andyzhangx]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx, groggemans

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