Golang cannot find package in Docker image - docker

Question

So, I am trying to dockerize a golang application with different directories containing supplementary code for my main file.
I am using gorilla/mux. The directory structure looks like this.
$GOPATH/src/github.com/user/server
|--- Dockerfile
|--- main.go
|--- routes/
handlers.go
|--- public/
index.gohtml
It works on my host machine with no problem. The problem is that when I try to deploy the docker image it does not run and exits shortly after creation. I have tried changing the WORKDIR command in my dockerfile to /go/src and dump all my files there, but still no luck. I have also tried the official documentation on docker hub. Doesn't work either.
My Dockerfile.
FROM golang:latest
WORKDIR /go/src/github.com/user/server
COPY . .
RUN go get -d github.com/gorilla/mux
EXPOSE 8000
CMD ["go","run","main.go"]
My golang main.go
package main
import (
"github.com/gorilla/mux"
"github.com/user/server/routes"
"log"
"net/http"
"time"
)
func main(){
//...
}
I get this error message when I check the logs of my docker image.
Error Message
main.go:5:2: cannot find package "github.com/user/server/routes" in any of:
/usr/local/go/src/github.com/user/server/routes (from $GOROOT)
/go/src/github.com/user/server/routes (from $GOPATH)

Answer 1

Try the following Docker file:
# GO Repo base repo
FROM golang:1.12.0-alpine3.9 as builder
RUN apk add git
# Add Maintainer Info
LABEL maintainer="<>"
RUN mkdir /app
ADD . /app
WORKDIR /app
COPY go.mod go.sum ./
# Download all the dependencies
RUN go mod download
COPY . .
# Build the Go app
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main .
# GO Repo base repo
FROM alpine:latest
RUN apk --no-cache add ca-certificates curl
RUN mkdir /app
WORKDIR /app/
# Copy the Pre-built binary file from the previous stage
COPY --from=builder /app/main .
# Expose port 8000
EXPOSE 8000
# Run Executable
CMD ["./main"]
Here, we are creating an intermediate docker builder container, copying the code into it, build the code inside the builder container and then copy the binary image to the actual docker.
This will help in both having all the dependencies in the final container and also, the size of the final image will be very small