Docker: use a entrypoint script (#1227)

In favor of `/etc/bash.bashrc`, which is only read for interactive
shells.

docker/entrypoint.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -e
+
+# Set-up PATH and other environment variables
+EMSDK_QUIET=1 source /emsdk/emsdk_env.sh
+
+exec "$@"