Upgrade Dockerfile base image from ubi7minimal to ubi8minimal

From operator-sdk v0.11.0 version the base image
changed from ubi7-minimal to ubi8-minimal.
operator-framework/operator-sdk#1952
from registry.access.redhat.com/ubi7/ubi-minimal:latest to
registry.access.redhat.com/ubi8/ubi-minimal:latest

build/Dockerfile

@@ -1,4 +1,4 @@
-FROM registry.access.redhat.com/ubi7/ubi-minimal:latest
+FROM registry.access.redhat.com/ubi8/ubi-minimal:latest
 
 ENV OPERATOR=/usr/local/bin/3scale-operator \
     USER_UID=1001 \