feat(constraints): Supports constraints for docker backend

[samber]

Related to #311 #342

[docker]
endpoint = "unix:///var/run/docker.sock"
constraints = ["tag==api", "tag==he*ld"]

$ docker run -d -P --label traefik.tags=api,helloworld emilevauge/whoami

[emilevauge]

ok == false => !ok
or even

if ok, failingConstraint := provider.MatchConstraints(constraintTags); !ok {
...
}

[emilevauge]

One minor comment.
Other than that, LGTM :)
Thanks @samber !
/cc @vdemeester

[vdemeester]

LGTM 🐼 with @emilevauge's comment taken care of and a rebase 👼

[tiagostutz]

Humm, I'm trying to use the suggested syntax but it is not working. Error log:

Error reading TOML config file /etc/traefik/traefik.toml : Type mismatch for 'main.TraefikConfiguration.Docker': Type mismatch for 'provider.Docker.Constraints': Type mismatch for types.Constraint. Expected map but found 'string'.

But I found a workaround (maybe a workaround) configuring the Constraints globally, described at #451.

Can you point what's the correct TOML syntax to configure constraints inside [docker] section?

Thanks!