Add abililty to use system python

Moving virtual environments between systems is problematic... and
unnecessary when creating container images. These modifications allow
the ability to install app dependencies into the system python, and use
that instead of a virtual environment.

script/bootstrap

@@ -9,13 +9,24 @@ set -e
 # Ensure we are in the app root directory (not the /script directory)
 cd "$(dirname "${0}")/.."
 
+if [ -z "${CIBUILD+xxxx}" ]; then
+  CMD_PREFIX='pipenv run '
+fi
+PIP_CMD="${CMD_PREFIX}pip"
+WEBASSETS_CMD="${CMD_PREFIX}webassets"
+
+PIPENV_INSTALL_FLAGS='--dev'
+if ${CIBUILD}; then
+  PIPENV_INSTALL_FLAGS+=' --system --ignore-pipfile'
+fi
+
 # Install Python dependencies
 pipenv --python 3.6
-pipenv run pip install --upgrade pip
-pipenv install --dev
+${PIP_CMD} install --upgrade pip
+pipenv install ${PIPENV_INSTALL_FLAGS}
 
 # Install uswds node module and dependencies
 npm install
 
 # Precompile assets for deployment
-pipenv run webassets -m atst.assets build
+${WEBASSETS_CMD} -m atst.assets build