build: setup docker-compose.dev.yml dev iteration

This is based off of the 5-commit branch at
https://github.com/neobooru/szurubooru/blob/docker-development-setup.

Compared to said branch, we
* Exclude extraneous changes such as
    * Any formatting
    * The use of deprecated/ineffectual top-level `version:` in composer files
* Support controlling $THREADS (modernizing the branch to upstream)
* Integrate into master more cleanly

However, client/docker-start-dev uses a temporary hack -- due to
volume mounting overwriting node_modules at arbitrary points during the
`docker compose build` step, we run `npm i` before any given
`npm run watch`.

To see the effects of this commit in action, run:

    docker compose -f ./docker-compose.dev.yml up

client/docker-start-dev.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/dumb-init /bin/sh
+
+# Integrate environment variables
+sed -i "s|__BACKEND__|${BACKEND_HOST}|" \
+  /etc/nginx/nginx.conf
+
+# Start server
+nginx &
+
+# Watch source for changes and build app
+# FIXME: It's not ergonomic to run `npm i` outside of the build step.
+#        However, the mounting of different directories into the
+#        client container's /opt/app causes node_modules to disappear
+#          (the mounting causes client/Dockerfile's RUN npm install
+#           to silently clobber).
+#        Find a way to move `npm i` into client/Dockerfile.
+npm i && npm run watch -- --polling