Can't automatically run simple Go web server with Docker container (func (*Template) Execute)

So I am trying to automatically run a simple "hello world" web server in a docker container on CoreOS. But I get an error when the app tries to exectute the HTML template.

Here is the offending code:

func GateHandler(w http.ResponseWriter, r *http.Request) {
    fmt.Println("Entered the GateHandler.")
    t, _ := template.ParseFiles("templates/helloworld.html")
    fmt.Println("Passed the ParseFiles.")
    err := t.Execute(w, nil)
    fmt.Println("Passed the Execute statement.")
    if err != nil {
        fmt.Println(err)
    }
}

Here is my Dockerfile:

FROM ubuntu:14.04
RUN mkdir app
ADD assets /app/assets 
ADD templates /app/templates
ADD web /app/web
ENV PORT 80
EXPOSE 80
ENTRYPOINT ["/app/web"]

When I run the docker container and navigate to the appropriate URL in my browser, I see the following error:

Entered the GateHandler.
Passed the ParseFiles.
2015/03/28 00:10:53 http: panic serving 10.0.2.2:56292: runtime error: invalid memory address or nil pointer dereference
goroutine 5 [running]:
net/http.func·011()
    /usr/local/go/src/net/http/server.go:1130 +0xbb
html/template.(*Template).escape(0x0, 0x0, 0x0)
    /usr/local/go/src/html/template/template.go:56 +0x3a
html/template.(*Template).Execute(0x0, 0x7f1593124360, 0xc20804c460, 0x0, 0x0, 0x0, 0x0)
    /usr/local/go/src/html/template/template.go:75 +0x3d
main.GateHandler(0x7f1593124290, 0xc20804c460, 0xc20803c1a0)

... and so on.

However, when I remove the entry point from the Dockerfile, run the container with /bin/bash and manually launch the app "./app/web", the web app runs perfectly. Looks like t.Execute() is not passing up an error like it should. The only difference is that I am launching the web app from docker run as opposed to manually starting it within the container.

Any insights? It would be a lot more convenient to launch the container and not have to worry about starting the web server manually.

Thanks.

<h3>UPDATE:</h3>

Being new to Go, I ignored the fundamental principle of never ignoring returned errors. Fixing the code gave more useful information.

Entered the GateHandler.
open templates/helloworld.html: no such file or directory
Passed the ParseFiles.
... and so on

So this means that when I automatically run the web app with the Docker container, it can't find the template file. Still working on figuring out why that is.

golang-nuts post: https://groups.google.com/forum/#!topic/golang-nuts/j6JTFpGg6fI

It was a working directory issue. Adding the following to the Dockerfile prior to the ENTRYPOINT statement fixed it.

WORKDIR /app

Previously, the app was trying to run in the root directory of the container and was unable to find the template file with the relative path in the GateHandler.

https://docs.docker.com/reference/builder/#workdir