set default for FILESDIR

[eshellman]

from lvl on DP forum:

Not sure if this is relevant, but I'm receiving a weird message: "WARNING:root:Not configured, using /var/www for FILESDIR"
Also, I had two img with alt="" and did not receive any INFO message.
ebookmaker output here: https://ebookmaker.pglaf.org/cache/2024 ... output.txt

Ebookmaker is complaining when FILESDIR is not present in the environment. It's not relevant outside of production, so a sensible default should be silently set.