On Writing Dockerfiles
I came across an email yesterday before I went home in our internal Docker mailing list. The author was looking for a Tomcat container written using SLES as a base-image. I didn’t remember coming across anything like that so I checked dockerhub. There were several there, but most of them, including the official one from Apache, were build on Debian or Ubuntu. I found one that uses a binary package in a tarball created by Apache. I created a plain container shell:
docker run -it sles12sp2 /bin/bash
Then I went through the dockerfile line by line making sure it worked. It didn’t right away. Suse products tend to put things into their own paths rather than the ones that Ubuntu/Debian uses and I fixed the dockerfile accordingly. It only took 20 minutes or so to test, build, and get it running.
I published the final version here using OpenSUSE instead of SLES. It’s also on dockerhub as: jsevans/tomcat-opensuse. This is how I use open source software. I take what others who are smarter than I am have made, build on it to make something better or at least different, and then give back to the community. Maybe my little hack will help someone else also.
One more thing.
Why didn’t I just add “zypper in tomcat” to my dockerfile? I tried that at first. However, like many Linux distros that moved to SystemD, services like Tomcat will not start without a real instance of SystemD running already. Some distros include that into their images like Ubuntu. Others like SLES/OpenSUSE and CentOS/Fedora don’t so you have to find alternative ways to install some applications.
That leads be to ask, will we see a day soon when applications are created to be “container ready”? The applications could be included in a container as is without needing to worry about dependencies to system processes like SystemD.