use player_id for session instead of UID. add build dockerfile

extension/AttendanceTracker/build/Dockerfile.build

@@ -0,0 +1,11 @@
+# build Golang app for Linux
+FROM golang:1.20
+
+WORKDIR /app
+
+# get gcc-multilib and gcc-mingw-w64
+RUN apt update
+RUN apt install -y gcc-multilib gcc-mingw-w64
+
+CMD ["/bin/sh"]
+