Chalk extract doesn't differentiate between docker image and file

## Description

If you have a path that happens to be in the same format as a Docker image chalk will select the file.

```
$ ~/chalk extract localhost:5000/wordsmith:latest
warn: /home/admin/wordsmith/api/localhost:5000/wordsmith:latest: Artifact is unchalked
info: /home/admin/.local/chalk/chalk.log: Open (sink conf='default_out')
info: Full chalk report appended to: ~/.local/chalk/chalk.log
... omitted
```

```
admin@ubuntu:~/wordsmith/api$ ls
Dockerfile localhost:5000 pom.xml src
admin@ubuntu:~/wordsmith/api$ ls localhost\:5000/
wordsmith:latest
```

Perhaps better to have a flag that signals that this is a docker image? Or a docker:// prefix to show that you are intended this to be a docker image?

## Impact

Dunno.

Start with the `chalk extract` command and reproduce the collision using a path named like `localhost:5000/wordsmith:latest`, as shown in the issue. Determine how the CLI distinguishes Docker images from filesystem paths, then confirm the chosen interface handles both cases without selecting the wrong artifact.

Written by the indexing model from the issue text.