`apk add` inherits stdin when called.

src/main.cr

@@ -127,7 +127,12 @@ begin
 
 	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 " "}"
+		opts = ["add"] + (ENV["APK_FLAGS"]? || "-i").split(/[ \t]+/) + dependencies
+		puts "+ apk #{opts.join " "}"
+		r = Process.run "apk", opts,
+			output: Process::Redirect::Inherit,
+			input: Process::Redirect::Inherit,
+			error: Process::Redirect::Inherit
 		if r.exit_status != 0
 			STDERR.puts "!! Running apk failed!"
 			exit 8