Warum schlägt meine Dockerized Go Web App mit der Fehlermeldung „Keine solche Datei oder kein solches Verzeichnis' fehl?

package main

import (
    "fmt"
    "net/http"
    "os"
)

func hostHandler(w http.ResponseWriter, r *http.Request) {
    name, err := os.Hostname()

    if err != nil {
        panic(err)
    }

    fmt.Fprintf(w, "<h1>HOSTNAME: %s</h1><br>", name)
    fmt.Fprintf(w, "<h1>ENVIRONMENT VARS: </h1><br>")
    fmt.Fprintf(w, "<ul>")

    for _, evar := range os.Environ() {
        fmt.Fprintf(w, "<li>%s</li>", evar)
    }
    fmt.Fprintf(w, "</ul>")

}

func rootHandler(w http.ResponseWriter, r *http.Request) {

    fmt.Fprintf(w, "<h1>Awesome site in Go!</h1><br>")
    fmt.Fprintf(w, "<a href='/host/'>Host info</a><br>")

}

func main() {

    http.HandleFunc("/", rootHandler)
    http.HandleFunc("/host/", hostHandler)
    http.ListenAndServe(":8080", nil)

}

FROM scratch
WORKDIR /home/ubuntu/go
COPY webapp /
EXPOSE 8080
CMD ["/webapp"]

CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -tags netgo -ldflags '-w' -o mybin *.go