Singularity %files behavior change in 3.6 and later

The behavior of the Singularity %files directive changed beginning in version 3.6. See https://github.com/hpcng/singularity/issues/5514.

For this Singularity definition file:

Bootstrap: docker
From: ubuntu:18.04

%files
   foo /var/tmp

%post
   ls -l /var/tmp/foo

Singularity 3.5.3:

# singularity --version
singularity version 3.5.3

# singularity build foo.sif Singularity.def 
INFO:    Starting build...
INFO:    Copying foo to /tmp/rootfs-f4ee8c55-7790-11eb-b63f-0242ac110002/var/tmp
INFO:    Running post scriptlet
+ ls -l /var/tmp/foo
-rw-r--r--. 1 root root 0 Feb 25 17:43 /var/tmp/foo
INFO:    Creating SIF file...
INFO:    Build complete: foo.sif

Singularity 3.6.4:

# singularity --version
singularity version 3.6.4

# singularity build foo.sif Singularity.def 
INFO:    Starting build...
INFO:    Copying foo to /tmp/build-temp-392680558/rootfs/var/tmp
INFO:    Running post scriptlet
+ ls -l /var/tmp/foo
ls: cannot access '/var/tmp/foo': No such file or directory

Changing the path from /var/tmp to /opt works with either version:

INFO:    Copying foo to /tmp/build-temp-697021000/rootfs/opt
INFO:    Running post scriptlet
+ ls -l /opt/foo
-rw-r--r--. 1 root root 0 Feb 25 17:41 /opt/foo
INFO:    Creating SIF file...
INFO:    Build complete: foo.sif

The --working-directory=/other/path option can be used to generate a Singularity definition file that works around this behavior change.

A better solution might be to use %setup to stage files into the container build environment. Alternatively, use a default location other than /var/tmp or /tmp when copying files from the host (such as the package option in many building blocks).

Originally reported in #341 by @VI-gha

The issue names no repository files, tests, or entry points. Start by reproducing the shown Singularity 3.5.3 and 3.6.4 builds with the example definition, then trace how host files and destinations are generated. Done should mean the generated definition handles the reported /var/tmp behavior across the affected Singularity versions without regressing the existing /opt case.