Remove healthcheck script for Docker image (#8095)

There are multiple possibilities for running MinIO within
a container e.g. configurable address, non-root user etc.
This makes it difficult to identify actual IP / Port to
use to check healthcheck status from within a container.

It is simpler to use external healthcheck mechanisms
like healthcheck command in docker-compose to check
for MinIO health status. This is similar to how checks
work in Kubernetes as well.

This PR removes the healthcheck script used inside
Docker container and ad documentation on how to
use docker-compose based healthcheck mechanism.

Dockerfile

@@ -9,8 +9,7 @@ ENV GO111MODULE on
 RUN  \
      apk add --no-cache git && \
      git clone https://github.com/minio/minio && cd minio && \
-     go install -v -ldflags "$(go run buildscripts/gen-ldflags.go)" && \
-     cd dockerscripts; go build -tags kqueue -ldflags "-s -w" -o /usr/bin/healthcheck healthcheck.go
+     go install -v -ldflags "$(go run buildscripts/gen-ldflags.go)"
 
 FROM alpine:3.9
 
@@ -22,7 +21,6 @@ ENV MINIO_ACCESS_KEY_FILE=access_key \
 EXPOSE 9000
 
 COPY --from=0 /go/bin/minio /usr/bin/minio
-COPY --from=0 /usr/bin/healthcheck /usr/bin/healthcheck
 COPY dockerscripts/docker-entrypoint.sh /usr/bin/
 
 RUN  \
@@ -33,6 +31,4 @@ ENTRYPOINT ["/usr/bin/docker-entrypoint.sh"]
 
 VOLUME ["/data"]
 
-HEALTHCHECK --interval=1m CMD healthcheck
-
 CMD ["minio"]