A new installation of the site no longer loads any CSS due to not having a default_skin_id set in the AdminSettings. I suspect caused by where default settings were changed but not codified in DB initialization scripts or startup rake tasks.
Run reset_database.sh (or rebuild environment completely)
Navigate to the dev site, note that no CSS is displayed
Verify via DB or rails console that default_skin_id is null in AdminSettings
Workaround is to set the default_skin_id either via rails console or just directly in the DB (remember to clear cache).
Suggestion from @tickinginstant:
It seems like it'd be cleaner to set it as part of the skins:load_site_skins task, which is run in the script/reset_database.sh script (which I believe is used by docker). I think it might work to add something like this to the task:
In a newly-built image, AdminSetting.first.default_skin_id is set to 1, and the local site has the default CSS. Looks good.