This commit changes the container base image
from ubi-minimal to ubi-micro.

The docker build process happens now in two stages.
The build stage:
 - downloads the latest CA certificate bundle
 - downloads MinIO binary (for requested version/os/arch)
 - downloads MinIO binary signature and verifies it
   using minisign

Then it creates an image based on ubi-micro with just
the minio binary was downloaded and verified during the
build stage.

The build stage is simplified to just verifying the
minisign signature.

Signed-off-by: Andreas Auernhammer <github@aead.dev>