dockerhub: elm_fates build fail due to invalid ssh setup

[glemieux]

Build fail log: https://hub.docker.com/repository/registry-1.docker.io/ngeetropics/elmtest/builds/1a615460-71c4-41c5-8b19-13a46100fbf2

It appears that the automated build is failing due to the fact that the elm_fates dockerfile was setup for local builds using DOCKER_BUILDKIT=1 experimental feature allowing for the ssh key mount type:

Encountered error: 400 Client Error: Bad Request ("Dockerfile parse error line 54: Unknown flag: mount")

This was setup so as to enable build secrets (particularly for handling personal SSH keys on the build machine). Reference information is here. It is possible to use advanced options for autobuild (and autotest) to allow scripts to run during the build process that will set necessary variables. An example script using DOCKER_BUILDKIT=1 gleaned from a google search is shown here.

That said, is this strictly necessary? What happens if we drop the usage of that option? It'd be nice to hang on to it so that a single dockerfile works for local builds and repo autobuilds for the time being.