switch minio container base image to ubi-mciro (#18329)

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>

dockerscripts/verify-minio.sh

@@ -1,31 +0,0 @@
-#!/bin/sh
-#
-
-set -e
-
-if [ ! -x "/opt/bin/minio" ]; then
-	echo "minio executable binary not found refusing to proceed"
-	exit 1
-fi
-
-verify_sha256sum() {
-	echo "verifying binary checksum"
-	echo "$(awk '{print $1}' /opt/bin/minio.sha256sum)  /opt/bin/minio" | sha256sum -c
-}
-
-verify_signature() {
-	if [ "${TARGETARCH}" = "arm" ]; then
-		echo "ignoring verification of binary signature"
-		return
-	fi
-	echo "verifying binary signature"
-	minisign -VQm /opt/bin/minio -P RWTx5Zr1tiHQLwG9keckT0c45M3AGeHD6IvimQHpyRywVWGbP1aVSGav
-}
-
-main() {
-	verify_sha256sum
-
-	verify_signature
-}
-
-main "$@"