Deprecate auto detection of container user (#7930)

There is no reliable way to handle fallbacks for
MinIO deployments, due to various command line
options and multiple locations which require
access inside container.

Parsing command line options is tricky to figure
out which is the backend disk etc, we did try
to fix this in implementations of check-user.go
but it wasn't complete and introduced more bugs.

This PR simplifies the entire approach to rather
than running Docker container as non-root by default
always, it allows users to opt-in. Such that they
are aware that that is what they are planning to do.

In-fact there are other ways docker containers can
be run as regular users, without modifying our
internal behavior and adding more complexities.

Dockerfile

@@ -10,8 +10,7 @@ RUN  \
      apk add --no-cache git && \
      git clone https://github.com/minio/minio && cd minio && \
      go install -v -ldflags "$(go run buildscripts/gen-ldflags.go)" && \
-     cd dockerscripts; go build -tags kqueue -ldflags "-s -w" -o /usr/bin/healthcheck healthcheck.go && \
-     go build -tags kqueue -ldflags "-s -w" -o /usr/bin/check-user check-user.go
+     cd dockerscripts; go build -tags kqueue -ldflags "-s -w" -o /usr/bin/healthcheck healthcheck.go
 
 FROM alpine:3.9
 
@@ -23,7 +22,6 @@ EXPOSE 9000
 
 COPY --from=0 /go/bin/minio /usr/bin/minio
 COPY --from=0 /usr/bin/healthcheck /usr/bin/healthcheck
-COPY --from=0 /usr/bin/check-user /usr/bin/check-user
 COPY dockerscripts/docker-entrypoint.sh /usr/bin/
 
 RUN  \