Add armv7 image for docker

The base image for the Dockerfile is `phusion/baseimage:master`. When I build the docker image it says that an armv7 version cannot be found. Please can you consider compiling an armv7 version of this base image? Pretty sure the rest of the Dockerfile will build just fine otherwise.

Start by inspecting the repository's Dockerfile and its phusion/baseimage:master reference, then check how that base image is built and whether armv7 support is provided upstream. Done means an armv7-compatible base image is available and the repository's Docker build completes successfully on armv7.

Written by the indexing model from the issue text.