Offer prebuilt ARM6 images for docker

[mhansen]

Hi, I'd like to ask for ARM6 images. I can make my own but would prefer to have these provided so they're always up to date. This is for running on a Raspberry Pi Zero W.

Something like this commit should work, just add ARM6: 666dbc7

$ uname -a
Linux zerow 4.14.98+ #1200 Tue Feb 12 20:11:02 GMT 2019 armv6l GNU/Linux

$ docker run prom/blackbox-exporter
Expected: runs, gives some logs
Exits with no logs - architecture mismatch, because the image only has an ARM7 version.

[mhansen]

Looks like we'd also depend on a busybox-linux-armv6 available at https://quay.io/organization/prometheus, because the Dockerfile builds inside busybox for the specified architecture: https://github.com/prometheus/blackbox_exporter/blob/master/Dockerfile#L3.

I wonder though, does it have to be this way? Go supports cross-compilation pretty well: you don't need to build inside an ARMv6 container to get an ARMv6 go binary.