Decrease final image size

[fabiendupont]

With the current Dockerfile, based on ubi8/ubi, the final image weighs 273 MB.
I have run some tests with other flavors of UBI:

• With ubi8/ubi-minimal, the final image weighs 146 MB.
• With ubi8/ubi-micro, the final image weighs 77 MB.

The size of the image doesn't have an impact on performance, but will save space both in the registry and on the nodes where pods run.

The downside of ubi8/ubi-micro is that it doesn't ship the tar command that is used by must-gather to copy files from an existing pod with kubectl cp. However, if must-gather doesn't need to pull files from the pod, tar becomes optional.