Make builder image more cache-friendly

Dockerfile

@@ -1,15 +1,13 @@
 FROM golang:1.12-alpine AS builder
-WORKDIR /src
-COPY . .
 RUN apk add -U --no-cache \
         build-base \
         ca-certificates \
         git \
         sqlite \
-        taglib-dev && \
+        taglib-dev
-    ./_do_build_server && \
+WORKDIR /src
-    ./_do_build_scanner && \
+COPY . .
-    apk del build-base
+RUN ./_do_build_server && ./_do_build_scanner
 
 FROM alpine
 COPY --from=builder \