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/js/main.js

@@ -3,7 +3,7 @@
 const config = require("./config.js");
 
 if (config.environment == "development") {
-    var ws = new WebSocket("ws://" + location.hostname + ":8080");
+    var ws = new WebSocket("ws://" + location.hostname + ":8081");
     ws.addEventListener("open", function (event) {
         console.log("Live-reloading websocket connected.");
     });