Primitive Docker and `OCI` label helpers for `RustUse`.
This crate validates label keys and values and exposes helpers for common `OCI`
annotation labels.
```rust
use use_docker_label::{DockerLabel, DockerLabelKey, OCI_TITLE};
let label = DockerLabel::new(DockerLabelKey::new(OCI_TITLE)?, "RustUse app")?;
assert_eq!(label.to_string(), "org.opencontainers.image.title=RustUse app");
```