Docker ONBUILD COPY doesn't seem to copy files

Question

I am trying to copy files to a docker image when I execute the docker build command. I am not sure what I am doing wrong because this seems to work for the docker rails onbuild file but doesn't work for my custom dockerfile.

Here is my Dockerfile

FROM ubuntu:14.04

RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

ONBUILD COPY Gemfile /usr/src/app

CMD ["tail", "-f", "/dev/null"]

The docker commands that I run are the following.

docker build -t copy-test .
docker run --name run-test -d copy-test
docker exec -i -t 2e9adebae0fc /bin/bash

When I connect to the container with docker exec it starts in /usr/src/app but the Gemfile is not there. I don't understand why the mkdir and WORKDIR instructions seem to work but the ONBUILD COPY does not. (And yes the directory I am invoking these commands in does have a Gemfile present)

Answer 1

You have to use COPY to build your image. Use ONBUILD if your image is kind of template to build other images.

See Docker documentation:

The ONBUILD instruction adds to the image a trigger instruction to be executed at a later time, when the image is used as the base for another build. The trigger will be executed in the context of the downstream build, as if it had been inserted immediately after the FROM instruction in the downstream Dockerfile.