Explain the npm verbosity in the node image

Close: #57
Related: nodejs/docker-iojs#36

Signed-off-by: Hans Kristian Flaatten <[email protected]>

Author: Hans Kristian Flaatten

README.md

@@ -53,6 +53,45 @@ $ docker run -it --rm --name my-running-script -v "$PWD":/usr/src/app -w
 /usr/src/app node:4 node your-daemon-or-script.js
 ```
 
+## Verbosity
+
+By default Node.js Docker Image has NPM log verbosity set to `info` instead of
+the default `warn`. This is because of the way Docker is isolated from the host
+operating system and you are not guaranteed to be able to retrieve the
+`npm-debug.log` file when something breaks.
+
+There are some scenarios where this creates too much output and you can reset
+the verbosity of npm using the following techniques:
+
+### Dockerfile
+
+If you create your own `Dockerfile` which inherits from the `node` image you can
+simply use `ENV` to override `NPM_CONFIG_LOGLEVEL`.
+
+```
+FROM node
+ENV NPM_CONFIG_LOGLEVEL warn
+...
+```
+
+### Docker Run
+
+If you run the node image using `docker run˙ you can use the `-e` flag to
+override `NPM_CONFIG_LOGLEVEL`.
+
+```
+$ docker run -e NPM_CONFIG_LOGLEVEL=warn node ...
+```
+
+### NPM run
+
+If you are running npm commands you can use `--loglevel` to control the
+verbosity of the output.
+
+```
+$ docker run node npm --loglevel=warn ...
+```
+
 # Image Variants
 
 The `node` images come in many flavors, each designed for a specific use case.