split _do_build_server

Dockerfile

@@ -11,6 +11,7 @@ COPY go.mod .
 COPY go.sum .
 RUN go mod download
 COPY . .
+RUN ./_do_gen_assets
 RUN ./_do_build_server
 
 FROM alpine:3.12.3

_do_build_server

@@ -1,5 +1,4 @@
 #!/bin/sh
 # using sh, is run from alpine
 
-./_do_gen_assets
 go build -o gonic cmd/gonic/main.go