Use the B2 'list' endpoint to determine file ID (#8169)

- More effective deletion and checking for existence.
- Rever Dockerfile.
- Add a 'GOPROXY' to the Dockerfile to workaround Apache issues.

Dockerfile

@@ -5,6 +5,7 @@ LABEL maintainer="MinIO Inc <dev@min.io>"
 ENV GOPATH /go
 ENV CGO_ENABLED 0
 ENV GO111MODULE on
+ENV GOPROXY https://proxy.golang.org
 
 RUN  \
      apk add --no-cache git && \