makes install available for all users in docker image (#3202)

* makes install available for all users in docker image

moves the installation path from /root/.local to a
virtualenv. this way we still get the lightweight
multistage build without excluding non-root users.

* adds changelog entry for docker-image fix

A changelog entry has been added under the Integration
subheader

* changes dockerfile to use the venv activate script

we are now using the inbuilt venv activate script, as well
as explicitly mentioning the binary location in the entrypoint
cmd.

Co-authored-by: Nicolò <nicolo.intrieri@spinforward.it>
Co-authored-by: Cooper Lees <me@cooperlees.com>