docs: Add packages for building in Alpine Linux (dashpay#3115)

* Add Alpine packages to UNIX build doc

* Add Alpine packages required for building GUI

Author: nmarley

doc/build-unix.md

@@ -25,6 +25,11 @@ $ sudo dnf install gcc-c++ libtool make autoconf automake python3 cmake libstdc+
 $ pacman -S base-devel python3 cmake
 ```
 
+##### Alpine Linux:
+```sh
+$ sudo apk --update --no-cache add autoconf automake cmake curl g++ gcc libexecinfo-dev libexecinfo-static libtool make perl pkgconfig python3 patch linux-headers
+```
+
 ##### FreeBSD/OpenBSD:
 ```bash
 pkg_add gmake cmake libtool