`tar` will add files in the order of the filesystem. This can vary from one build to the next.

= Solution =

Instead of solely using `tar`, use `find+sort+tar`.

Before:

{{{
tar -cf archive.tar src
}}}

After for Tar version 1.28+:

{{{
tar --sort=name -cf archive.tar src
}}}

After for older versions of Tar:

{{{
find src -print0 | LC_ALL=C sort -z | tar --no-recursion --null -T - -cf archive.tar
}}}