I've observed that sometimes the overlay mount unit does not get started
when using wantedBy. requiredBy makes this relationship stricter and if
necessary will restart the initrd-fs.target and thus ensure that when
this target is reached /etc has alredy been mounted. This is in line
with the description of initrd-fs.target in systemd.special:
> Thus, once this target is reached the /sysroot/ hierarchy is fully set up
