Dockerfile: Install everything in one command

Prepend `apt install` command by `apt update` command in `Dockerfile`.

Signed-off-by: Razvan Deaconescu <razvan.deaconescu@upb.ro>

Dockerfile

 FROM gitlab.cs.pub.ro:5050/operating-systems/assignments-docker-base
 
-RUN apt update -yqq
-RUN apt install -yqq libaio-dev
-RUN apt install -yqq lsof
-RUN apt install -yqq netcat
+RUN apt update -yqq && \
+	 apt install -yqq libaio-dev lsof netcat
 
 COPY ./checker ${CHECKER_DATA_DIRECTORY}
 RUN mkdir ${CHECKER_DATA_DIRECTORY}/../tests