Fix issue with build arg in drone deployment (#1602)

The ARG before the FROM call in multistage builds is dropped with the
previous container. We need to define it globally

Signed-off-by: Chris M <millscj01@gmail.com>

cmd/proxy/Dockerfile

@@ -6,6 +6,8 @@
 # See project Makefile if using make.
 # See docker --build-arg if building directly.
 ARG GOLANG_VERSION=1.14
+ARG ALPINE_VERSION=3.11.5
+
 FROM golang:${GOLANG_VERSION}-alpine AS builder
 
 WORKDIR $GOPATH/src/github.com/gomods/athens
@@ -16,7 +18,6 @@ ARG VERSION="unset"
 
 RUN DATE="$(date -u +%Y-%m-%d-%H:%M:%S-%Z)" && GO111MODULE=on CGO_ENABLED=0 GOPROXY="https://proxy.golang.org" go build -ldflags "-X github.com/gomods/athens/pkg/build.version=$VERSION -X github.com/gomods/athens/pkg/build.buildDate=$DATE" -o /bin/athens-proxy ./cmd/proxy
 
-ARG ALPINE_VERSION=3.11.5
 FROM alpine:${ALPINE_VERSION}
 
 ENV GO111MODULE=on