A simple calculator made with Go

Dockerfile

FROM golang
RUN mkdir /build
ADD . /build/
WORKDIR /build
ADD . ./
RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -installsuffix cgo -ldflags="-w -s" -o gocalc .
ENTRYPOINT ["./gocalc"]

Dockerfile.multi-stage

FROM golang:onbuild as builder
RUN mkdir /build
ADD . /build/
WORKDIR /build
ADD . ./
RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -installsuffix cgo -ldflags="-w -s" -o gocalc .

FROM scratch
COPY --from=builder /build/gocalc /
ENTRYPOINT ["/gocalc"]

gocalc.go

package main
import (
	"errors"
	"fmt"
	"os"
	"strconv"
)

func calculate(e []string) (float64, error) {
	result := 0.0
	num1, err := strconv.ParseFloat(e[0], 64)
	if err != nil {
		return 0.0, err
	}
	num2, err := strconv.ParseFloat(e[2], 64)
	if err != nil {
		return 0.0, err
	}
    switch e[1] {
    case "*":
        result = num1 * num2
    case "/":
        if num2 == 0.0 {
            return 0.0, errors.New("error: you tried to divide by zero.")
        }
        result = num1 / num2
    case "+":
        result = num1 + num2
    case "-":
        result = num1 - num2
    default:
        return 0.0, errors.New("error: don't know how to do that")
    }
	return result, nil
}

func main() {
    if len(os.Args) != 4 {
        fmt.Println("error: wrong number of arguments")
        return
    }
    res, err := calculate(os.Args[1:])
    if err != nil {
        fmt.Println(err)
    } else {
        fmt.Println(res)
    }
}