doc: add alpine depends build instructions

depends/README.md

@@ -53,6 +53,18 @@ To build dependencies for the current arch+OS:
 
     gmake
 
+### Alpine
+
+    apk add bash build-base cmake curl make patch
+
+Skip the following packages if you don't intend to use the GUI and will build with [`NO_QT=1`](#dependency-options):
+
+    apk add bison linux-headers samurai pkgconf python3
+
+To build dependencies for the current arch+OS:
+
+    make
+
 ## Configuring Bitcoin Core
 
 **When configuring Bitcoin Core, CMake by default will ignore the depends output.** In