add a posit user to the base content container

When service accounts are in play, they default to UID/GID 999. For
instance, on Connect. We create a posit user in this place for consistency
across products, environments, etc.

content/base/Dockerfile.ubuntu1804

@@ -90,7 +90,7 @@ RUN apt-get update  \
 
 # Reference: https://docs.rstudio.com/resources/install-r/
 
-# We are NOT linking one of these R versions into the PATH.
+# We are NOT linking any of these R versions into the PATH.
 
 ARG R_DISTRIBUTION=ubuntu-1804
 
@@ -107,7 +107,7 @@ RUN curl -fsSL -O https://cdn.rstudio.com/r/${R_DISTRIBUTION}/pkgs/${R_INSTALLER
 
 # Reference: https://docs.rstudio.com/resources/install-python/
 
-# We are NOT linking one of these Python versions into the PATH.
+# We are NOT linking any of these Python versions into the PATH.
 
 ARG MINICONDA_VERSION=4.7.12.1
 ARG PYTHON_VERSION=3.7.6
@@ -124,9 +124,13 @@ RUN curl -fsSL -o miniconda.sh https://repo.anaconda.com/miniconda/Miniconda3-${
     # remove miniconda too, for size
     && rm -rf /opt/miniconda
 
-# install quarto
+# Install Quarto --------------------------------------------------------------#
 ARG QUARTO_VERSION=1.0.37
 COPY maybe_install_quarto.sh /tmp/maybe_install_quarto.sh
 RUN /opt/R/${R_VERSION}/bin/R -e 'install.packages("odbc", repos="https://packagemanager.rstudio.com/cran/__linux__/bionic/latest")' \
     && /tmp/maybe_install_quarto.sh \
     && rm -f /tmp/maybe_install_quarto.sh
+
+# Add a posit user / group --------------------------------------------------------------#
+RUN groupadd -g 999 -r posit && \
+	useradd -r -u 999 -g 999 -M posit