Deb: build-deb now builds a .deb for Ubuntu 18

It builds the builder image first, then proceeeds to building .deb
inside it, and the result is put into the parent (../) directory.

builder/dockerfile.ubuntu1804.deb.build

@@ -3,6 +3,8 @@ FROM ubuntu:18.04
 RUN apt-get update && \
       apt-get -y install vim build-essential devscripts equivs
 
+# Install build-deps for the package.
 COPY ./debian/control /tmp
+RUN echo YYY | mk-build-deps --install --remove /tmp/control
 
-RUN echo YYY | mk-build-deps --install /tmp/control
+USER 1000