mount go cache during docker build

Dockerfile

@@ -1,25 +1,21 @@
-FROM golang:1.14-alpine AS base
+FROM golang:1.14-alpine AS builder
 RUN apk add -U --no-cache \
-        build-base \
-        ca-certificates \
-        git \
-        sqlite \
-        taglib-dev
+    build-base \
+    ca-certificates \
+    git \
+    sqlite \
+    taglib-dev
 WORKDIR /src
 COPY go.mod .
 COPY go.sum .
-ENV GO111MODULE=on
 RUN go mod download
-
-FROM base AS builder
-WORKDIR /src
 COPY . .
 RUN ./_do_build_server && ./_do_build_scanner
 
-FROM alpine
+FROM alpine:3.9
 RUN apk add -U --no-cache \
-	ffmpeg \
-	ca-certificates
+    ffmpeg \
+    ca-certificates
 COPY --from=builder \
     /usr/lib/libgcc_s.so.1 \
     /usr/lib/libstdc++.so.6 \