Add sudo to tests/docker

This makes sudo available in the Docker-based test environment, in order
to make the environment slightly more similar to GitHub Actions (which
doesn't feature gosu).  With sudo it is easier to locally reproduce
sudo-related problems observed on GitHub actions.

tests/docker/Dockerfile

@@ -10,9 +10,12 @@ ENV DEBIAN_FRONTEND noninteractive
 ### Installing packages
 RUN sed -ie 's/^# *deb-src/deb-src/' /etc/apt/sources.list  # Enable source repos
 RUN apt-get update && \
-    apt-get install -y software-properties-common gosu && \
+    apt-get install -y software-properties-common gosu sudo && \
     chmod u+s /usr/sbin/gosu
 
+RUN echo "build   ALL =(ALL: ALL) NOPASSWD: ALL" > /etc/sudoers.d/navbuild
+
+
 RUN add-apt-repository ppa:deadsnakes/ppa && \
     apt-get update && \
     apt-get -y install --no-install-recommends \