Use same env var in Dockerfile and Makefile
Ensures that build tags get set in the Dockerfile so that OSS and GCS drivers
are built into the official registry binary.
Closes
FROM golang:1.13.4-alpine AS build
ENV DISTRIBUTION_DIR /go/src/github.com/docker/distribution
-ENV DOCKER_BUILDTAGS include_oss include_gcs
+ENV BUILDTAGS include_oss include_gcs
ARG GOOS=linux
ARG GOARCH=amd64
