add GNU parallel

[tomeichlersmith]

this will be helpful for running multiple instances of fire across the number of cores the container image has access to

I am adding a new package to the container, here are the details.

What new packages does this PR add to the development image?

• parallel

Check List

• I successfully built the container using docker

• I was able to build ldmx-sw using this new container build

• I was able to test run a small simulation and reconstruction inside this container

• I was able to successfully use the new packages.

• add parallel and explanation to list in docs