In part two, I said that the Dockerfile and fig nicely worked on my box in boot2docker.  Given that Docker exists to move things from environment to environment, I anticipated that that would mean the system would now work in our Jenkins on Linux continuous integration environment.  An error on my part…

It turns out that I’d downloaded a version of boot2docker which supplied Docker 1.5.  That’s a very recent release of Docker, it turns out, just released in February.  The Jenkins server is running Docker 1.3, as the CentOS repos it uses don’t yet have newer versions.  Docker 1.3.2 was released last November, with 1.3.1 in October.  I’m not sure sitting here at home which minor version the Jenkins server is running, but what I am sure of is that the behavior of Dockerfile’s WORKDIR changed between the two.

Docker does a good job of making it easy to jump between the documentation supporting different versions.  In 1.3, the documentation for WORKDIR says it sets the working directory for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile.  In 1.5, WORKDIR also supports COPY and ADD.  My Dockerfile used ADD to bring specific files into the container, so that variance in behavior meant that my ENTRYPOINT couldn’t actually find the files I’d moved in.  It took me a while to figure out –  inspecting the docker logs had told me that the ENTRYPOINT wasn’t working, but not a whole lot more.  To debug it, I had to take out my ENTRYPOINT so that I could get the container to start up, and then go in with a `docker run -it -d [IMAGE] bash` to actually go look and see where things were.

Now, once I knew I had a WORKDIR behavior variance, I went looking at the release notes for Docker, from v1.3 through 1.5.  WORKDIR was mentioned, but only in the context of environment variable expansion.  I also went looking on GitHub for issues with information, and didn’t find anything that appeared relevant.  (There are 19 open and 68 closed issues related to WORKDIR, as of this evening.

I mostly spent the time writing this up moreso to catch my debugging path, and perhaps make this easier for someone else to find.  It also makes me consider digging into what support Docker provides for indicating the version of Docker that an image was built against.  We have `docker history` to see the list of commands used to build the layers.  We have `docker info` and `docker version` to pull the information about our own Docker environment.  Somehow annotating what Docker version a file has been tested against, and/or an image has been built with might be a very useful structure, in terms of determining the viability of the Dockerfile in another environment.  Beyond commands changing behavior, there is the very real likelihood that new Dockerfile commands will be added with new Docker releases.  Having some kind of concept of these match perfectly versus these might not match would be a good place to work.

Note: I did do a check out on registry.hub.docker.com to see how they handle support for differing versions of Docker. The official distributions of mongo, CentOS and redis tag their releases and provide links to their Dockerfiles, but just generally state ‘

This image is officially supported on Docker version 1.5.0.

Support for older versions (down to 1.0) is provided on a best-effort basis.’

I did use the mongo official release as part of my efforts: it itself seemed to do fine across 1.3 versus 1.5.  A quick peek into its Dockerfile shows me that I wouldn’t have tripped the WORKDIR issue, at least unless its debian:wheezy base image caused an issue.

Note: further poking shows that someone added an issue for Docker file version information about a year ago (March 29, 2014) in issue #4907.  Happily for my cause, it was tagged by a software engineer at Docker as a proposed feature just two weeks ago.