Resolve "bug: `images.txt` doesn't include manually specified images in `synker.yaml`"

Uses yq to strip the image list out of the synker file to create images.txt - previously only images that were run in the cluster appeared in this list.

I also lumped in one small change to remove the use of sed to add images to synker which was "stupid and hacky". Now uses yq and explicitly populates those images to the proper yaml block.

Closes #230 (closed)