Version V2 for yelb app server fails to run, missing gem rackup

[pishukla-aws]

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Issue #, if available:

Description of changes:

On going through the blog which was authored in the past regarding getting started with App Mesh, at the 5th step where a new version of yelb app server is build. The docker image fails to run because its missing a gem rackup.

Error from the pod

k logs yelb-appserver-v2-588786cd57-mdf59 -n yelb Defaulted container "yelb-appserver-v2" out of: yelb-appserver-v2, envoy, xray-daemon, proxyinit (init) Sinatra could not start, the "rackup" gem was not found!

Add it to your bundle with:

bundle add rackup

or install it with:

gem install rackup

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I made the change in the dockerfile and was able to deploy it successfully . This PR is for include the gem in the package

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