Drop arm32v6 and arm32v7 builds for now

[yosifkit]

arm32v6: still broken

+ bashbrew build docker:20.10.8
...
+ dockerd --version
Bus error (core dumped)

+ bashbrew build docker:19.03.15
...
+ dockerd --version
Segmentation fault (core dumped)

arm32v7: still broken

+ Bus error docker:20.10.8
...
+ dockerd --version
Illegal instruction (core dumped)

+ bashbrew build docker:19.03.15
...
+ dockerd --version
Segmentation fault (core dumped)

ref #260
closes #325

cc @thaJeztah anything @tianon and I can do to help debug the upstream docker builds for arm32*?

[tianon]

Going to merge this for now -- I love this implementation because it leaves the Dockerfile bits as-is so that this is easier to test again later to revalidate. 😄