Can't `go get` dependencies during docker build - docker

Question

I'm fairly new to both Docker and go, so this might be something obvious, but my google searches haven't found anything.
I'm trying to build a simple go program with docker, but I'm havign trouble with dependencies.
go file:
package main
import (
"fmt"
"log"
"html"
"net/http"
"github.com/gorilla/mux"
)
func hello(writer http.ResponseWriter, r *http.Request) {
path := mux.Vars(r)["rest"]
fmt.Fprintf(writer, "Hello, %q", html.EscapeString(path))
}
func main() {
router := mux.NewRouter().StrictSlash(true)
router.HandleFunc("/{rest:.*}", hello)
log.Println("Listening...")
log.Fatal(http.ListenAndServe(":8080", router))
}
Docker file:
FROM golang:latest
RUN mkdir /app
ADD ./HelloWorld.go /app/
WORKDIR /app
RUN go get ./*.go
RUN go build -o main .
CMD ["/app/main"]
Error:
Sending build context to Docker daemon 6.482MB
Step 1/7 : FROM golang:latest
---> 138bd936fa29
...
Step 5/7 : RUN go get ./*.go
---> Running in 1e29844961a2
HelloWorld.go:9:5: cannot find package "github.com/gorilla/mux" in any of:
/usr/local/go/src/github.com/gorilla/mux (from $GOROOT)
/go/src/github.com/gorilla/mux (from $GOPATH)
The command '/bin/sh -c go get ./*.go' returned a non-zero code: 1

Answer 1

You may use my Dockerfile as a base. First stage produces working image. That’s enough for many cases.
https://github.com/lisitsky/go-site-search-string/blob/light-docker/Dockerfile
If you want to shrink image size from ~800MB to about ~10-20MB use second stage too.

Answer 2

Just use $GOPATH everywhere in your path to build your images, e.g. $GOPATH/src/app instead of /app
Full example of multistage build:
FROM golang:alpine as builder
RUN apk add --update git
RUN mkdir -p $GOPATH/src/build
ADD . $GOPATH/src/build/
WORKDIR $GOPATH/src/build
RUN go get ./...
RUN go build -o main .
FROM alpine
RUN adduser -S -D -H -h /app appuser
USER appuser
COPY --from=builder /go/src/build/main /app/
WORKDIR /app
CMD ["./main"]