Perform word splitting on custom arguments (#560)

Custom arguments set via the Docker environment file are passed to the
entrypoint script as a single string (rather than anything more structured like
an array). Use xargs to split this string into shell words, enabling the passing
of custom arguments which individually contain whitespace (such as
`--telemetry-url 'wss://telemetry.polkadot.io/submit/ 1'`).

Fixes Cardinal-Cryptography/aleph-node-runner#14.

docker/docker_entrypoint.sh

@@ -110,8 +110,4 @@ if [[ -n "${UNIT_CREATION_DELAY:-}" ]]; then
   ARGS+=(--unit-creation-delay="${UNIT_CREATION_DELAY}")
 fi
 
-if [[ -n "${CUSTOM_ARGS:-}" ]]; then
-  ARGS+=(${CUSTOM_ARGS})
-fi
-
-aleph-node "${ARGS[@]}"
+echo "${CUSTOM_ARGS}" | xargs aleph-node "${ARGS[@]}"