pkg/units: Refactor regexp.Compile to init()
pkg/units: Refactor regexp.Compile to init()

No need to recompile a fixed regular expression each time the function
executes. Abstracting it to the `init()` method.

Docker-DCO-1.1-Signed-off-by: Francisco Carriedo <fcarriedo@gmail.com> (github: fcarriedo)

docker/go-units