git checkout Tag 2.7.3

reg-app/Dockerfile

 FROM alpine/git AS clone
 WORKDIR /app
 RUN git clone https://git.scc.kit.edu/simon/reg-app.git
+WORKDIR /app/reg-app
+RUN git checkout tags/2.7.3 -b branch-2.7.3
 
 FROM maven:3.6-openjdk-11-slim AS build