Fix installation of local plugins (fixes #6912)

[CristianCantoro]

The background for this pull request is contained in #6912.

This pull request does 3 things:

1. creates a directory local_plugins in the root repository, with a .gitignore file that ignores all of its contents.
2. creates a script bin/installLocalPlugins.sh.
3. modifies the Dockerfile to change the installation process of local plugins:
   • copies the contents of the local_plugins directory into the container (if the directory does not exist, it skips it).
   • defines a new build arg variable ETHERPAD_LOCAL_PLUGINS_ENV
   • install the local plugins with the script bin/installLocalPlugins.sh (which uses pnpm install -w "local_plugins/${plugin}/")

(note: I am not sure that the pnpm -w install command is the best way to install a plugin, I am not an expert node user.)

To install a plugin locally put the it inside the local_plugins directory and pass the name of the directory ETHERPAD_LOCAL_PLUGINS, like this (full log: build-pr-1.log):

docker build --no-cache --progress=plain --target production --build-arg ETHERPAD_LOCAL_PLUGINS="ep_disable_reset_authorship_colours" -t cristiancantoro/etherpad-lite:2.2-fix6912 .

(I am using the ep_disable_reset_authorship_colours as an example)

When running the container with the following command:

docker run -it -p 9001:9001 -v ./copy:/tmp/copy -e ADMIN_PASSWORD=foo cristiancantoro/etherpad-lite:2.2-fix6912

The log shows the following line:

[2025-03-23T19:41:57.719] [INFO] server - Installed plugins: ep_disable_reset_authorship_colours@0.0.25

(full log: run-pr-1.log)

[SamTV12345]

Thanks for the fix. I'll merge when the pipeline is happy :)