Change default differ from apt to file

[aaron-prindle]

Currently container-diff defaults to the apt differ, I think that this is confusing for users as running container-diff diff <image-1> <image-2> will say that there is no difference in the images (referring to no different apt-packages) when in fact files might be different and it isn't obvious that the default is apt for new users.

[dlorenc]

The reason we didn't do file by default is that the output can be huge. Not sure what the best approach is.

[nkubala]

I would be fine removing the default in favor of making the user specify at least one diff type. I agree with @aaron-prindle that defaulting to apt can be confusing but we definitely don't want to default to file, especially for the analyze command

[peter-evans]

It's not merged yet but --type=size might be a good candidate for the default.
See #256

Removing the default altogether, as @nkubala suggested, would also be fine I think.

[peter-evans]

#256 has been merged. What do you think about making it the default? This is what it would look like:

$ container-diff analyze gcr.io/gcp-runtimes/diff-layer-base

-----Size-----

Analysis for gcr.io/gcp-runtimes/diff-layer-base:
IMAGE                                      DIGEST                                                                         SIZE
gcr.io/gcp-runtimes/diff-layer-base        sha256:3327d2f4a099a1aaa530c1cd93453ac0a3556f3eff616e9ec86751a0c8ac3f27        19B

$ container-diff diff gcr.io/gcp-runtimes/diff-layer-base gcr.io/gcp-runtimes/diff-layer-modified

-----Size-----

Image size difference between gcr.io/gcp-runtimes/diff-layer-base and gcr.io/gcp-runtimes/diff-layer-modified:
SIZE1        SIZE2
19B          15B

[nkubala]

@peter-evans I think it's probably fine, I don't think there's any "right" answer here :) would you like to open a PR?