Add Alpine build instructions

Author: hoxell

README.md

@@ -99,6 +99,22 @@ pkg install devel/cmake devel/boost-libs textproc/expat2 \
   databases/postgresql94-client graphics/proj lang/lua52
 ```
 
+On Alpine, use
+
+```sh
+apk --update-cache add cmake make g++ boost-dev expat-dev \
+  bzip2-dev zlib-dev libpq proj-dev lua5.3-dev
+```
+
+Due to Lua installation details, CMake only detects some of the
+required Lua paths on Alpine, but not the library.
+This can be resolved by setting the `LUA_DIR` environment variable,
+or you can modify `CMAKE_PREFIX_PATH` as mentioned below.
+
+```sh
+export LUA_DIR=<path>  # Defaults to /usr/lib/lua5.3/
+```
+
 Once dependencies are installed, use CMake to build the Makefiles in a separate
 folder: