Add Dockerfile to run example

[roaslin]

Added dockerfile.dev and docker-compose.yml to run the example locally

I can't really test the checkout because I don't have a Sandbox account API key.

Issue

I ran into this error while running the example

            npm ERR! code EACCES
npm ERR! syscall mkdir
npm ERR! path /home/blah/projects/example-web-checkout/node_modules/body-parser
npm ERR! errno -13
npm ERR! Error: EACCES: permission denied, mkdir '/home/blah/projects/example-web-checkout/node_modules/body-parser'
npm ERR!  [Error: EACCES: permission denied, mkdir '/home/blah/projects/example-web-checkout/node_modules/body-parser'] {
npm ERR!   errno: -13,
npm ERR!   code: 'EACCES',
npm ERR!   syscall: 'mkdir',
npm ERR!   path: '/home/blah/projects/example-web-checkout/node_modules/body-parser'
npm ERR! }
npm ERR! 
npm ERR! The operation was rejected by your operating system.
npm ERR! It is likely you do not have the permissions to access this file as the current user
npm ERR! 
npm ERR! If you believe this might be a permissions issue, please double-check the
npm ERR! permissions of the file and its containing directories, or try running
npm ERR! the command again as root/Administrator.

Solves

• Running into permissions errors when running npm i
• Avoid installing npm modules on local machine