Add info log during init step prior to each download of an image for visibility

[ecrupper]

Description

I've noticed this being a more relevant issue when using vela exec pipeline. Essentially, if you don't have the image in your local Docker cache, then the pipeline hangs at

[step: init] > Preparing service images...
[step: init] > Preparing stage images...
[step: init] > Preparing step images...

While it downloads the first image. It follows it up with an inspect:

[step: init] $ docker image inspect golang:latest
sha256:0c4ed86491a49bc887be0382d454058ab30887445728dd1fdb346ebc69395fe9

So the simple request is adding a line like

[step: init] > Preparing step images...
[step: init] > Downloading golang:latest...

# long pause happens here

[step: init] $ docker image inspect golang:latest
sha256:0c4ed86491a49bc887be0382d454058ab30887445728dd1fdb346ebc69395fe9

Value

People may prematurely cancel their local runs, thinking there is something wrong, as it's not intuitive what's going on, especially if the first image to download is hefty.

Definition of Done

Informative log added prior to each image download during init step

Effort (Optional)

Low