Add a way to use private repositories on GitHub (#869)

entrypoint.sh

@@ -4,4 +4,8 @@
 # provides all arguments concatenated as a single string.
 ARGS=("$@")
 
+if [[ ! -z "${GITHUB_AUTHENTICATION_TOKEN}" ]]; then
+  git config --global --add url."https://x-access-token:${GITHUB_AUTHENTICATION_TOKEN}@github.com/".insteadOf "https://github.com/"
+fi
+
 /bin/gosec ${ARGS[*]}