/bin/sh error when trying to build Ruby image with Docker

I’m working on creating a Docker image that can be used to run a suite of Ruby scripts. I’m just testing things out for now, and have a Dockerfile like this:

FROM alpine:3.2

RUN mkdir -p /usr/src/app
ADD . /usr/src/app

WORKDIR /usr/src/app

CMD "ruby test.rb"

The test.rb file is literally just a puts "hello" statement.

I run docker build -t docker_test . and then when I run docker run docker_test I get this error: /bin/sh: ruby test.rb: not found.

I get the same error if I install the official Ruby docker image, and if I try to include other installations. Am I missing something here? I would provide more information if possible, but I genuinely haven’t seen this issue come up in any of the tutorials or documentation for Docker.

Source: StackOverflow