Add package description for deb

Avoid build warning that gets confused with the output file name:

    ENOENT: no such file or directory, stat 'deb-warning: description field
    is missing in Cargo.toml'

runtime/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.3.0"
 authors = ["Golem Factory <[email protected]>"]
 edition = "2021"
 license = "GPL-3.0"
+description = "Docker-like runtime environment for Linux systems"
 
 [package.metadata.deb]
 assets = [