added missing build step to access admin end

[mwanjajoel]

This pull request fixes the error Error: ENOENT: no such file or directory, open '/srv/app/build/index.html' when strapi is run in production.

The cause of the error is the missing build step for the admin dashboard and the fix is to add the command bash -c "strapi build && strapi start"

[strapi-cla]

All committers have signed the CLA.

[drullandev]

okay