nginx? Boo.. Caddy? Yay!
Please note this was not of my own creation. It comes from Naoki Kosaka's https://gist.github.com/yukimochi/bb7c90cbe628f216f821e835df1aeac1
If you are finding your asset / static files are all 404 with this, this can help:
usermod -aG mastodon caddy
Alternatively, you could get Mastodon's Rails process to serve static files itself, but they advise against this for performance reasons.