checkout main branch instead of master

Author: angelo-v

test/surface/docker/server/Dockerfile

@@ -1,7 +1,7 @@
 FROM node:latest
 RUN git clone https://github.com/solid/node-solid-server
 WORKDIR node-solid-server
-RUN git checkout master
+RUN git checkout main
 RUN npm install
 RUN openssl req -new -x509 -days 365 -nodes \
   -out ./server.cert \