feature(entrypoint): use entrypoint script to launch cmd

- allows preservation of ctrl-c termination
- 'shell' command as proxy for /bin/bash

entrypoint.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+#
+# ab-dart-sass entrypoint script
+#
+
+if [ "$1" = "shell" ]; then
+    exec /bin/bash
+elif [ -n "$1" ]; then
+    exec "$@"
+else
+    exec /opt/dart-sass/sass -s "$SASS_STYLE" --watch --poll --stop-on-error /sass:/css
+fi
+
+exit $?
+
+#EOF