removed the ENV for DEBIAN_FRONTEND=noninteractive and use export instead

Dockerfile

@@ -1,20 +1,20 @@
-# VERSION 0.1
+# VERSION 0.2
 # AUTHOR:         Miroslav Prasil <miroslav@prasil.info>
 # DESCRIPTION:    Image with DokuWiki & lighttpd
 # TO_BUILD:       docker build -t mprasil/dokuwiki .
 # TO_RUN:         docker run -d -p 80:80 --name my_wiki mprasil/dokuwiki
 
 
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 MAINTAINER Miroslav Prasil <miroslav@prasil.info>
 
 # Set the version you want of Twiki
-ENV DOKUWIKI_VERSION=2018-04-22b
-ARG DOKUWIKI_CSUM=605944ec47cd5f822456c54c124df255
+ENV DOKUWIKI_VERSION=2018-04-22c
+ARG DOKUWIKI_CSUM=6272e552b9a71a764781bd4182dd2b7d
 ARG DOKUWIKI_DEBIAN_PACKAGES=""
 
 # Update & install packages & cleanup afterwards
-RUN export DEBIAN_FRONTEND=noninteractive && \
+RUN DEBIAN_FRONTEND=noninteractive && \
     apt-get update && \
     apt-get -y upgrade && \
     apt-get -y install \
@@ -52,4 +52,3 @@ VOLUME ["/dokuwiki/data/","/dokuwiki/lib/plugins/","/dokuwiki/conf/","/dokuwiki/
 
 ENTRYPOINT ["/startup.sh"]
 CMD ["run"]
-