Add README for example

Signed-off-by: Vladimir Aleksandrov <valeksan@redhat.com>

Author: derasdf

README.md

@@ -1,4 +1,65 @@
-# Repo of examples for Hermeto docs
+# npm example
 
-Individual examples each live in their own branch (e.g. the basic `pip` example is in
-the 'pip-basic' branch)
+This example demonstrates using Hermeto with a simple Node.js app that uses npm for dependency management.
+
+## Pre-fetch dependencies
+
+The `package-lock.json` file in this repo specifies which npm packages to download.
+Run Hermeto to pre-fetch the dependencies into a local output directory:
+
+```shell
+hermeto fetch-deps --source . --output ./hermeto-output '{"type": "npm"}'
+```
+
+## Generate environment variables
+
+Generate an environment file so you can pass the right settings to `npm install`:
+
+```shell
+hermeto generate-env ./hermeto-output -o ./hermeto.env --for-output-dir /tmp/hermeto-output
+```
+
+Currently Hermeto does not require any environment variables for npm, but this
+may change in the future, so it is recommended to always generate this file.
+
+## Inject project files
+
+Next, inject the project files so that remote npm tarball URLs are rewritten to
+point at the locally fetched artifacts:
+
+```shell
+hermeto inject-files ./hermeto-output --for-output-dir /tmp/hermeto-output
+```
+
+After this step, entries in `package-lock.json` like:
+
+```diff
+-      "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.8.tgz",
++      "resolved": "file:///tmp/hermeto-output/deps/npm/accepts-1.3.8.tgz",
+```
+
+will point to files inside the mounted Hermeto output directory instead of the network.
+
+## Build the application image
+
+The `Containerfile` in this repo uses the `node:18` base image and runs `npm install`
+inside the container. With the injected files and environment file in place, the
+install step can run without network access.
+
+Build the image while mounting the Hermeto data:
+
+```shell
+podman build . \
+  --volume "$(realpath ./hermeto-output)":/tmp/hermeto-output:Z \
+  --volume "$(realpath ./hermeto.env)":/tmp/hermeto.env:Z \
+  --network none \
+  --tag npm-basic-example
+```
+
+## Run the container
+
+```shell
+podman run -it --init -p 9000:9000 npm-basic-example
+```
+
+Then open `http://localhost:9000` in your browser to reach the app.