Shift "ubuntu" user removal to base build.

Author: alexmv

Dockerfile

@@ -13,14 +13,13 @@ RUN { [ ! "$UBUNTU_MIRROR" ] || sed -i "s|http://\(\w*\.\)*archive\.ubuntu\.com/
     apt-get -q update && \
     apt-get -q dist-upgrade -y && \
     DEBIAN_FRONTEND=noninteractive \
-        apt-get -q install --no-install-recommends -y ca-certificates git locales python3 sudo tzdata
-
+    apt-get -q install --no-install-recommends -y ca-certificates git locales python3 sudo tzdata && \
+    touch /var/mail/ubuntu && chown ubuntu /var/mail/ubuntu && userdel -r ubuntu && \
+    useradd -d /home/zulip -m zulip -u 1000
 
 FROM base AS build
 
-RUN touch /var/mail/ubuntu && chown ubuntu /var/mail/ubuntu && userdel -r ubuntu && \
-    useradd -d /home/zulip -m zulip -u 1000 && \
-    echo 'zulip ALL=(ALL:ALL) NOPASSWD:ALL' >> /etc/sudoers
+RUN echo 'zulip ALL=(ALL:ALL) NOPASSWD:ALL' >> /etc/sudoers
 
 USER zulip
 WORKDIR /home/zulip