I worked in a library for almost 8 years as a PHP developer until 3 years ago and never heard of this PHP package since we didn’t need it. I don’t think too many people use it besides programmers in libraries. So the chance is small that you can find a public Docker image containing File_MARC. I would say that you can try it, but I am sure you have already tried :).
Docker Compose is to run containers and not to install packages. You can create your own Dockerfile and build it with Docker Compose
I didn’t really use PEAR much so I don’t know the best way to install it or if it is still recommended to use it (official php images contain pear 1), but I found it on packagist. If you use composer for your dependencies, I recommend you to use this version. If you don’t use composer, well… then I recommend you to use it