VPN Unlimited support (#499)

- Fixes #420 
- Revert to docker/build-push-action@v2.4.0

internal/provider/vpnunlimited/provider.go

@@ -0,0 +1,19 @@
+package vpnunlimited
+
+import (
+	"math/rand"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+type Provider struct {
+	servers    []models.VPNUnlimitedServer
+	randSource rand.Source
+}
+
+func New(servers []models.VPNUnlimitedServer, randSource rand.Source) *Provider {
+	return &Provider{
+		servers:    servers,
+		randSource: randSource,
+	}
+}