Update documentation for elyra development image

etc/docker/elyra_development/Dockerfile

@@ -21,8 +21,7 @@ RUN wget --quiet https://repo.anaconda.com/miniconda/Miniconda3-4.5.11-Linux-x86
 
 # update nodejs
 RUN curl -fsSL https://deb.nodesource.com/setup_16.x | bash - && \
-    apt-get install -y nodejs && \
-    rm setup_16.x
+    apt-get install -y nodejs
 
 # install yarn
 RUN curl -sL https://dl.yarnpkg.com/debian/pubkey.gpg | gpg --dearmor | tee /usr/share/keyrings/yarnkey.gpg >/dev/null && \

etc/docker/elyra_development/README.md

@@ -0,0 +1,60 @@
+<!--
+{% comment %}
+Copyright 2018-2022 Elyra Authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+{% endcomment %}
+-->
+
+### Elyra Development Container Image
+
+Builds an Elyra development workspace image. This image provides all the necessary libraries and frameworks to build and test
+Elyra changes. 
+
+#### Building the image
+
+To build a custom version of this container image:
+1. Clone this repository
+   ```
+   $ git clone https://github.com/elyra-ai/elyra.git
+   ```
+1. [Optional] Add custom Python package requirements to `requirements.yml` in the `path_to_elyra_repo_root/etc/docker/elyra_development` directory.
+
+1. Build the image using the `Dockerfile` in the `path_to_elyra_repo_root/etc/docker/elyra_development` directory, replacing the `<path_to_elyra_repo_root>`, `<image_name>`, and `<tag>` placeholders as appropriate.
+   ```
+   $ docker build -t <image_name>:<tag> <path_to_elyra_repo_root>/etc/docker/elyra_development/
+   ``` 
+
+#### Using this image
+This image mounts your local Elyra repository into a directory in the image (`/dev/elyra`) so all changes are persisted. 
+
+In a terminal window run the following command, replacing the `<image_name>`, `<tag>`, and `<host_fs_elyra_repo>` placeholders as appropriate.
+
+```
+$ docker run -v <host_fs_elyra_repo>:/dev/elyra -p 8888:8888 -it <image_name>:<tag>
+```
+
+**Example**
+
+Assuming the container image was built and named `test_user/elyra-environment:dev` and 
+the cloned Elyra repository is located at `/path_to_elyra_repo_root`, run
+
+```
+$ docker run -v /path_to_elyra_repo_root:/dev/elyra -p 8888:8888 -it test_user/elyra-environment:dev
+```
+
+#### Starting Elyra in the container
+Once inside the container, after building with your changes in Elyra, start JupyterLab with:
+```bash
+$ jupyter lab --allow-root --ip=0.0.0.0 --debug
+```