build: install GTK without development files

README.md

@@ -108,8 +108,16 @@ export XMODIFIERS=@im=strans
 xterm
 ```
 
-For GTK 3 applications, install with normal `DESTDIR` semantics.  A staged
-install does not update the host GTK module cache:
+For GTK 3 applications, install the module built by Docker.  Installation
+needs the GTK 3 runtime, but not its development package:
+
+```sh
+doas make -C gtk install
+```
+
+`LIBDIR` defaults to `/usr/lib` and may be overridden for another system.
+Normal `DESTDIR` semantics are supported, and a staged install does not update
+the host GTK module cache:
 
 ```sh
 make -C gtk install DESTDIR="$pkgdir"