Some Observations About Installing Mastodon on Arch.
Nginx
From the Production Guide
you can copy the example nginx.conf file to /etc/nginx/sites-enabled/some_arbitrary.conf,
and then add the following to /etc/nginx/nginx.conf in the http section,
this with a fresh install of nginx with the default configuration file.
# /etc/nginx/nginx.conf 
http {
    include sites-enabled/*;
}
Installing the Dependancies
pacman -S certbot nginx libxml2 imagemagick ffmpeg git yarn npm python2 oidentd
# I'm guessing here
pacman -S libpqxx libxslt protobuf protobuf-c
- I'm assuming base-devel is installed
 - python2 seems to be required to run 
yarn installcommand later on - oidentd seems to be a usable replacement for pident
 - libpqxx pulls in postgresql-libs
 - file is already installed
 - curl is already installed
 - ruby-build and rbenv are installable from aur
 - also postgresql and redis unless, those are in another container or whatever.
 
Other Observations
I discovered that between gem install bundler and
bundle install --deployment --without development test,
you have to update your environment, with 
eval "$(rbenv init -)", i.e.
echo 'eval "$(rbenv init -)"' >> .bashrc
# and then
. ~/.bashrc
You have to update your environment more than once, during the installation.
Presumably you don't ever want to delete the ~/live/Public/ directory
if that is where assets are being stored, but it seems ok to delete 
~/live/node_modules and then rerun the yarn install command.
In ~/live/.env.production, SINGLE_USER_MODE=false has to be set
to false until at least one user is created, or the web service won't 
even start.  (Also chmod 755 ~/)