Dynamically load Element Web modules in Docker entrypoint (#29346) (#29358)

* Dynamically load Element Web modules in Docker entrypoint



* Iterate



* Drop environment for PR runs



* Iterate



---------



(cherry picked from commit 8ef84349b5)

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

docs/install.md

@@ -66,6 +66,18 @@ on other runtimes may require root privileges. To resolve this, either run the
 image as root (`docker run --user 0`) or, better, change the port that nginx
 listens on via the `ELEMENT_WEB_PORT` environment variable.
 
+[Element Web Modules](https://github.com/element-hq/element-modules/tree/main/packages/element-web-module-api) can be dynamically loaded
+by being made available (e.g. via bind mount) in a directory within `/tmp/element-web-modules/`.
+The default entrypoint will be index.js in that directory but can be overridden if a package.json file is found with a `main` directive.
+These modules will be presented in a `/modules` subdirectory within the webroot, and automatically added to the config.json `modules` field.
+
+If you wish to use docker in read-only mode,
+you should follow the [upstream instructions](https://hub.docker.com/_/nginx#:~:text=Running%20nginx%20in%20read%2Donly%20mode)
+but additionally include the following directories:
+
+- /tmp/element-web-config/
+- /etc/nginx/conf.d/
+
 The behaviour of the docker image can be customised via the following
 environment variables: