Macro dockerfile_rs::FROM

macro_rules! FROM {
    ($image:ident) => { ... };
    ($image:ident AS $name:ident) => { ... };
    ($image:ident:$tag:ident) => { ... };
    ($image:ident:$tag:ident AS $name:ident) => { ... };
    ($image:ident@$digest:ident) => { ... };
    ($image:ident@$digest:ident AS $name:ident) => { ... };
}

let from = FROM!(rust:latest);
assert_eq!(from.to_string(), "FROM rust:latest");