feat: Support docker-like references

[jssblck]

Overview

Adds support for docker-like references.
The following all work:

docker pull ubuntu
docker pull contribsys/faktory
docker pull docker.io/ubuntu
docker pull library/ubuntu

Previously, circe would error on these cases; now it doesn't, and works as you'd expect.
In effect, partially-qualified references are expanded to docker.io/library/{name} by default.

The choice of docker.io being the default was chosen mainly for compatibility with the behavior in FOSSA CLI. I considered adding more hosts, but decided it'd be better to push users to fully qualify their references instead of having them lean on potentially surprising fallback behavior.

Acceptance criteria

Docker-style pulls are possible.

Testing plan

Automated tests cover this mostly; did some basic manual tests below:

; cargo run -- extract contribsys/faktory scratch --overwrite
   Compiling circe_lib v0.3.3 (/Users/jess/projects/circe/lib)
   Compiling circe v0.3.3 (/Users/jess/projects/circe/bin)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.52s
     Running `target/debug/circe extract contribsys/faktory scratch --overwrite`
┐main{opts=Options { target: Target { image: "contribsys/faktory", platform: None, username: None, .. }, output_dir: "scratch", overwrite: true, layers: Squash, layer_glob: None, file_glob: None, layer_regex: None, file_regex: None }}
├─  INFO extracting image
├─  WARN expanding 'contribsys/faktory' to 'docker.io/contribsys/faktory'; fully specify the reference to avoid this behavior
├─  INFO removing existing output directory, path="/Users/jess/projects/circe/scratch"
├─  INFO enumerated 6 layers
├─  INFO applying layer 1 of 6, layer=sha256:cf04c63912e16506c4413937c7f4579018e4bb25c272d989789cfba77b12f951
├─  INFO applying layer 2 of 6, layer=sha256:c7d295e5ea131404d7daad6c665c069eaca393373338ad18a14b90556f0540f5
├─  INFO applying layer 3 of 6, layer=sha256:6fbf45066bb4f8b01361714c3110fa6963bc3b5a7b1e525802f204283a3b20b4
├─  INFO applying layer 4 of 6, layer=sha256:1704d03cf7438aa26afa5705246511247b3530b64ff596114aea2a5ac7bd7ef0
├─  INFO applying layer 5 of 6, layer=sha256:f5a817431cb3b1f5362e01b289f7989be55e37b9e9baaec262263078eedf7676
├─  INFO applying layer 6 of 6, layer=sha256:a124f83435a22b4e87a7d4e544a7f0b3273e06d196c181cc3b1b7945961640d1
├─  INFO finished applying layers
┘

; cargo run -- extract ubuntu scratch --overwrite                            
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running `target/debug/circe extract ubuntu scratch --overwrite`
┐main{opts=Options { target: Target { image: "ubuntu", platform: None, username: None, .. }, output_dir: "scratch", overwrite: true, layers: Squash, layer_glob: None, file_glob: None, layer_regex: None, file_regex: None }}
├─  INFO extracting image
├─  WARN expanding 'ubuntu' to 'docker.io/library/ubuntu'; fully specify the reference to avoid this behavior
├─  INFO removing existing output directory, path="/Users/jess/projects/circe/scratch"
├─  INFO enumerated 1 layer
├─  INFO applying layer 1 of 1, layer=sha256:8bb55f0677778c3027fcc4253dc452bc9c22de989a696391e739fb1cdbbdb4c2
├─  INFO finished applying layers
┘

; cargo run -- extract docker.io/ubuntu scratch --overwrite
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running `target/debug/circe extract docker.io/ubuntu scratch --overwrite`
┐main{opts=Options { target: Target { image: "docker.io/ubuntu", platform: None, username: None, .. }, output_dir: "scratch", overwrite: true, layers: Squash, layer_glob: None, file_glob: None, layer_regex: None, file_regex: None }}
├─  INFO extracting image
├─  WARN expanding 'docker.io/ubuntu' to 'docker.io/library/ubuntu'; fully specify the reference to avoid this behavior
├─  INFO removing existing output directory, path="/Users/jess/projects/circe/scratch"
├─  INFO enumerated 1 layer
├─  INFO applying layer 1 of 1, layer=sha256:8bb55f0677778c3027fcc4253dc452bc9c22de989a696391e739fb1cdbbdb4c2
├─  INFO finished applying layers
┘

Metrics

None

Risks

Allowing users to provide partially qualified references introduces potential for confusion into the system, but since this is the same behavior as docker (with which most container users are familiar) I think this is minimized.

Checklist

• I added tests for this PR's change (or explained in the PR description why tests don't make sense).

[csasarak]

LGTM.

[Optional, probably more relevant for the CLI and super minor] It might be worth making the "base" configurable via env var. If I'm a company who has a list of image names it may be simpler for me to set an env var specifying the base rather than have to program my CI jobs to concatenate it together with the base name. It also means that I can create a CI job template that sets the var and that makes calls to this do the right thing rather than making my engineers have to remember to use the FQN.

^^ I'm a little dubious that this is super useful, but I think it's worth explicitly rejecting.

[jssblck]

that's an incredible idea! made that quick change.

[csasarak]

I was thinking about this more over lunch, and it would be similar to also have CLI options that can do that.

[jssblck]

IMO this is behavior that we probably don't want in flags; it's not really intended to be used most of the time, it's just an escape hatch.

[csasarak]

Makes sense, thanks!