`apk` is run to install build-dependencies.

src/main.cr

@@ -76,6 +76,23 @@ end
 
 begin
 	latest_build_dir = ""
+
+	dependencies = [] of String
+	recipes.each do |recipe|
+		recipe.build_dependencies.each do |dep|
+			dependencies << dep unless dependencies.any? &.==(dep)
+		end
+	end
+
+	if dependencies.size > 0
+		# FIXME: We’ll probably want to have backends other than apk at some point.
+		r = context.sh "apk add #{ENV["APK_FLAGS"]? || "-i"} #{dependencies.join " "}"
+		if r.exit_status != 0
+			STDERR.puts "!! Running apk failed!"
+			exit 8
+		end
+	end
+
 	recipes.each do |recipe|
 		latest_build_dir = recipe.building_directory