Fix Docker Build/Push

[dduportal]

The last release v0.1.0 failed to build the Docker image: https://github.com/updatecli/updatecli/runs/2014386430?check_suite_focus=true.

This PR aims at fixing these issues, following the work done in #188 .

It introduces the following changes:

• Introduce a build arg to the docker image to specify the make target to use in the "builder" image. It defaults to build on a PR, but should be build.all on a tagged build (need to be tested with a tag build)
• Build the Docker images even on Pull Requests to catch build errors earlier (the condition of "push image if not a pull request is already present)
• Disable the arm build which was failing the release: the goal would be to improve the build process in the future and re-enable arm