Updated Nginx Docker Image to alpine version

[Madhu1512]

This PR updates is to add Nginx alpine docker image instead of Nginx jessie image. This will reduce the docker image size to half.

[codesuki]

Thanks for your pull request. It is certainly a good idea to reduce the docker image size. But maybe this should be a separate docker file so people can choose which release they use.
Apart from that some changes in your pull request seem to have broken the thing.
By adding ["nginx", "-g", "daemon off;"] nginx becomes blocking and the ecs-gen command does not run. Also I am not sure you can use the [] syntax together with the shell syntax, but I can't say that for sure.

[Madhu1512]

Agreed. Let me make the changes and submit another pull request