several small updates for trusty and xenial

- no longer need stringr: docopt 0.6.0 removed the Depends
- no longer set default CRAN repo as Debian/Ubuntu's R now does it

r-apt/trusty/Dockerfile

@@ -2,8 +2,10 @@
 
 FROM ubuntu:trusty
 
-## This handle reaches Carl and Dirk
-MAINTAINER "Carl Boettiger and Dirk Eddelbuettel" rocker-maintainers@eddelbuettel.com
+LABEL org.label-schema.license="GPL-2.0" \
+      org.label-schema.vcs-url="https://github.com/rocker-org/r-base" \
+      org.label-schema.vendor="Rocker Project" \
+      maintainer="Dirk Eddelbuettel <edd@debian.org>"
 
 ## Set a default user. Available via runtime flag `--user docker` 
 ## Add user to 'staff' group, granting them write privileges to /usr/local/lib/R/site.library
@@ -32,15 +34,14 @@ RUN echo "en_US.UTF-8 UTF-8" >> /etc/locale.gen \
 	&& /usr/sbin/update-locale LANG=en_US.UTF-8
 
 ## Now install R and littler, and create a link for littler in /usr/local/bin
-## Also set a default CRAN repo, and make sure littler knows about it too
-## Note 1: we need wget here as the build env is too old to work with libcurl (we think, precise was)
-## Note 2: r-cran-docopt is not currently in c2d4u so we install from source
+## Default CRAN repo is now set by R itself, and littler knows about it too
+## r-cran-docopt is not currently in c2d4u so we install from source
+## Note: we need wget as the build env is too old for libcurl (we think, precise was)
 RUN apt-get install -y --no-install-recommends \
                 r-cran-littler \
 		r-base \
 		r-base-dev \
 		r-recommended \
-                r-cran-stringr \
         && echo 'options(repos = c(CRAN = "https://cran.rstudio.com/"), download.file.method = "wget")' >> /etc/R/Rprofile.site \
 	&& ln -s /usr/lib/R/site-library/littler/examples/install.r /usr/local/bin/install.r \
 	&& ln -s /usr/lib/R/site-library/littler/examples/install2.r /usr/local/bin/install2.r \

r-apt/xenial/Dockerfile

@@ -2,8 +2,10 @@
 
 FROM ubuntu:xenial
 
-## This handle reaches Carl and Dirk
-MAINTAINER "Carl Boettiger and Dirk Eddelbuettel" rocker-maintainers@eddelbuettel.com
+LABEL org.label-schema.license="GPL-2.0" \
+      org.label-schema.vcs-url="https://github.com/rocker-org/r-base" \
+      org.label-schema.vendor="Rocker Project" \
+      maintainer="Dirk Eddelbuettel <edd@debian.org>"
 
 ## Set a default user. Available via runtime flag `--user docker` 
 ## Add user to 'staff' group, granting them write privileges to /usr/local/lib/R/site.library
@@ -33,19 +35,16 @@ RUN echo "en_US.UTF-8 UTF-8" >> /etc/locale.gen \
 ENV LC_ALL en_US.UTF-8
 ENV LANG en_US.UTF-8
 
-# Now install R and littler, and create a link for littler in /usr/local/bin
-# Also set a default CRAN repo, and make sure littler knows about it too
-# Note 1: we need wget here as the build env is too old to work with libcurl (we think, precise was)
-# Note 2: r-cran-docopt is not currently in c2d4u so we install from source
+## Now install R and littler, and create a link for littler in /usr/local/bin
+## Default CRAN repo is now set by R itself, and littler knows about it too
+## r-cran-docopt is not currently in c2d4u so we install from source
 RUN apt-get update \
         && apt-get install -y --no-install-recommends \
                  littler \
  		 r-base \
  		 r-base-dev \
  		 r-recommended \
-                 r-cran-stringr \
                  r-cran-rcpp \
-        && echo 'options(repos = c(CRAN = "https://cran.rstudio.com/"), download.file.method = "libcurl")' >> /etc/R/Rprofile.site \
  	&& ln -s /usr/lib/R/site-library/littler/examples/install.r /usr/local/bin/install.r \
  	&& ln -s /usr/lib/R/site-library/littler/examples/install2.r /usr/local/bin/install2.r \
  	&& ln -s /usr/lib/R/site-library/littler/examples/installGithub.r /usr/local/bin/installGithub.r \