Document things to avoid for building containers

[jscook2345]

This is the opposite of good practices for building containers. Anti-patterns and things to avoid.

[jscook2345]

Removing unknown files from /tmp can cause issues.

sylabs/singularity#3632

Instead, if you must delete unknown files under /tmp, maybe do something like setting TMPDIR for different parts of the container setup and delete that when finished.

[jscook2345]

See also: https://singularity.lbl.gov/docs-docker#best-practices