Introduce ubi-minimal:8-based Docker images

[axw]

Introduce another Docker image based on the RedHat Universal Base Image (minimal): https://catalog.redhat.com/software/containers/ubi8/ubi-minimal/5c359a62bed8bd75a2c3fba8. This would be in addition to our existing images.

It's likely that there will be little to no effort required specifically for APM Server, and this will simply be a matter of pulling in Beats changes.