Add README for web-plugin-packager (#2025)

- Provide instructions on how to build a custom docker image with JS plugins installed

docker/web-plugin-packager/README.md

@@ -0,0 +1,73 @@
+# web-plugin-packager
+
+web-plugin-packager is a docker image used to package JS plugins for the Deephaven Web UI with your own custom docker container.
+
+## Usage
+
+First, follow the instructions to [Launch Deephaven from pre-built images](https://deephaven.io/core/docs/tutorials/quickstart/#set-up-your-deephaven-deployment).
+
+Once you have the `docker-compose.yml` file pulled down, define your own web docker image in `web/Dockerfile` that includes the plugins you would like to use. 
+
+1. Create subdirectory `web` in the same folder as your `docker-compose.yml`: `mkdir web`
+2. Create the `Dockerfile` for web and open for editing: `vi web/Dockerfile`
+3. Paste the following into the `web/Dockerfile` and save:
+```
+# Pull the web-plugin-packager image
+FROM ghcr.io/deephaven/web-plugin-packager:latest as build
+
+# Specify the plugins you wish to use. You can specify multiple plugins separated by a space, and optionally include the version number, e.g.
+# RUN ./pack-plugins.sh <js-plugin-name>[@version] ...
+# For a list of published plugins, see https://www.npmjs.com/search?q=keywords%3Adeephaven-js-plugin
+# Here is how you would install the matplotlib and table-example plugins
+RUN ./pack-plugins.sh @deephaven/js-plugin-matplotlib @deephaven/js-plugin-table-example
+
+# Copy the packaged plugins over
+FROM ghcr.io/deephaven/web:latest
+COPY --from=build js-plugins/ /usr/share/nginx/html/js-plugins/
+```
+
+Many plugins will require a server side component as well. To define the plugins used on the server, create a `server/Dockerfile` similar to above:
+
+1. Create subdirectory `server` in the same folder as your `docker-compose.yml`: `mkdir server`
+2. Create the `Dockerfile` for server and open for editing: `vi server/Dockerfile`
+3. Paste the following into the `server/Dockerfile` and save:
+```
+FROM ghcr.io/deephaven/server:latest
+
+# pip install any of the plugins required on the server
+RUN pip install deephaven-plugin-matplotlib
+```
+
+After building, you need to specify using that build in your `docker-compose`. Do this by modifying the existing a `docker-compose.yml` file and replace the web and server definitions with the following:
+```yaml
+services:
+  server:
+    # Comment out the image name
+    # image: ghcr.io/deephaven/server:${VERSION:-latest}
+    # Build from your local Dockerfile you just created
+    build: ./server
+
+    ...
+
+  web:
+    # Comment out the image name
+    # image: ghcr.io/deephaven/web:${VERSION:-latest}
+    # Build from your local Dockerfile you just created
+    build: ./web
+```
+
+When you're done, your directory structure should look like:
+```commandline
+.
+├── docker-compose.yml
+├── server
+│   └── Dockerfile
+└── web
+    └── Dockerfile
+```
+
+Everything's ready to go! Now you just need to run `docker-compose up` as normal, and you will be using your custom image with your JS plugins installed. See what you can create!
+
+## JS Plugin Development
+
+For instructions on developing your own JS plugin, look at the [deephaven-js-plugin-template repository](https://github.com/deephaven/deephaven-js-plugin-template/).