include autopkgtest/build-essential to speed up and save bandwidth

This Docker image will be cached locally on the runners and often reused.

Dockerfile

@@ -14,6 +14,8 @@ RUN mkdir -p /usr/share/man/man1 \
 	&& apt-get -qy upgrade \
 	&& apt-get -qy dist-upgrade \
 	&& apt-get -qy install --no-install-recommends \
+		autopkgtest \
+		build-essential \
 		ca-certificates \
 		fakeroot \
 		git-buildpackage \