I wanted to do a rather simple thing in a dockerfile: read a value from an external file and use it in a RUN command.
Doing all this inside the dockerfile was a requirement, so I couldn’t pass the version while building the image using build-args or a similar solution.
There’s nothing hard about achieving this, as long as you keep something in mind: Docker creates intermediate containers
for a lot of things, including RUN commands. So that means that everything needs to be done in the same RUN pipeline.
The following example shows how we can read a version from an external file and export a variable that we can use (in the
same pipeline).