doc/npm.md: Update docs for Alpine Linux (#2479)

Author: Oxylibrium

doc/npm.md

@@ -4,6 +4,7 @@
 
 - [Ubuntu, Debian](#ubuntu-debian)
 - [Fedora, CentOS, RHEL](#fedora-centos-rhel)
+- [Alpine](#alpine)
 - [macOS](#macos)
 - [FreeBSD](#freebsd)
 
@@ -36,6 +37,13 @@ sudo yum install -y python2 libsecret-devel libX11-devel libxkbfile-devel
 npm config set python python2
 ```
 
+## Alpine
+
+```bash
+apk add alpine-sdk bash libstdc++ libc6-compat libx11-dev libxkbfile-dev libsecret-dev
+npm config set python python3
+```
+
 ## macOS
 
 ```bash