Dockerfile.armhf should be updated

[jrbenito]

Description of the issue

Dockerfile is now using python:3.6 as base image and Dockerfile.armhf still uses Debian as base image. Also, armhf version tries to build python what is not necessary since python official image has official support for ARM architecture.

Context information (for bug reports)

Not applicable

Steps to reproduce the issue

1. Just check Dockerfile and Dockerfile.armhf
2. Check python:3.6 official image for ARM support

Observed result

Outdated Dockerfile.armhf that may lead to different results from regular Dockerfile.

Expected result

ARM and X86 (and hopefully other archs) shall produce similar (if not identical) results.

Stacktrace / full error message

not applicable

Additional information

Not applicable.