Why am I getting 404 with Go in docker

This is my Dockerfile

FROM golang:alpine AS build
RUN apk --no-cache add gcc g++ make git
WORKDIR /go/src/app
COPY . .
RUN go get ./...
RUN GOOS=linux go build -ldflags="-s -w" -o ./bin/web-app ./main.go

FROM alpine:3.9
RUN apk --no-cache add ca-certificates
WORKDIR /usr/bin
COPY --from=build /go/src/app/bin /go/bin
EXPOSE 80
ENTRYPOINT /go/bin/web-app --port 80

And this is a simple main.go:

package main

import (
    "fmt"
    "net/http"
)

func main() {
    http.Handle("/", http.FileServer(http.Dir("./public")))
    log.Fatal(http.ListenAndServe(":80", nil))
}

The public directory is in the same folder with the main.go file. There is an index.html in the public directory.
When I run go run main.go my website is served properly. But with Docker, I get the 404 page. I suspect it has to do with the placement of the files inside the docker container. Any help is appreciated.

Source: StackOverflow