Fix Gem not found issue

[zhang4952]

Descriptive summary

Running the ED2 docker image has "Gem not found" error even after re-build docker image, and app fails to start.

Expected behavior

ED2 running at "localhost:3000" after "docker-compose up".

Actual behavior

• "gem not found" error appear while editing ruby code
• "localhost:3000" shows error of "no data return"

[zhang4952]

fix via #240