Add a function for getting uncompressed image size and show the size

[hhorak]

This might help when playing with container size changes. We can later add some check to see what is the difference between the current size and the last image available, but this is not implemented yet.

[hhorak]

[test]

[hhorak]

[test]

[hhorak]

[test]

[pkubatrh]

Could we also add compressed size as well? Might be good idea to list the image size at the end of the tests instead of at the start, so that we do not have to look through the log to find the info.

[hhorak]

@pkubatrh I've added the size to the end, that seems like a better place. I'm not sure though what's the best way to get the compressed size -- the only way I see when working with a local image is exporting to a tar and then gzip it, which does not seem to be what we actually need. Any suggestions?

[hhorak]

We discussed today on a meeting, that for approximate compressed size, we could likely export an image and gzip it.

[hhorak]

[test]

[hhorak]

[test]

[hhorak]

[test]

[hhorak]

[test]

[hhorak]

[test]

[hhorak]

[test]

[hhorak]

All feedback addressed, CI passes (except el7 that makes problems) but there is no risk of regression, so I believe we can merge now.