Use . instead of source

As Dockerfile executes with /bin/sh, not /bin/bash

repo2docker/buildpacks/r.py

@@ -263,7 +263,7 @@ class RBuildPack(PythonBuildPack):
                         libclang-dev \
                         libzmq3-dev > /dev/null && \
                 wget --quiet -O /tmp/r-{self.r_version}.deb \
-                    https://cdn.rstudio.com/r/ubuntu-$(source /etc/os-release && echo $VERSION_ID | sed 's/\.//')/pkgs/r-{self.r_version}_1_amd64.deb && \
+                    https://cdn.rstudio.com/r/ubuntu-$(. /etc/os-release && echo $VERSION_ID | sed 's/\.//')/pkgs/r-{self.r_version}_1_amd64.deb && \
                 apt install --yes --no-install-recommends /tmp/r-{self.r_version}.deb > /dev/null && \
                 rm /tmp/r-{self.r_version}.deb && \
                 apt-get -qq purge && \