Output the size of the images being pushed to the image registry in the build process

[seanhamlin]

We are looking to solve some performance issues around image pushes from the Lagoon docker host to the image registry and the time it takes. In order to help with this it would be good to emit the size of the images in the dockerhost, prior to the push in the build log.

e.g.

Image master/solr:latest	238.52MB
Image master/php:latest		435.58MB
Image master/nginx:latest	147.05MB
Image master/cli:latest		248.02MB
Image Size Total			1069.17MB

Using this information we can then approximate the MB/sec for the image push (the time it takes to do this is already in the build log).