VPN Unlimited support (#499)

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

internal/updater/providers/vpnunlimited/resolve.go

@@ -0,0 +1,32 @@
+package vpnunlimited
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/updater/resolver"
+)
+
+func resolveHosts(ctx context.Context, presolver resolver.Parallel,
+	hosts []string, minServers int) (hostToIPs map[string][]net.IP,
+	warnings []string, err error) {
+	const (
+		maxFailRatio    = 0.1
+		maxDuration     = 20 * time.Second
+		betweenDuration = time.Second
+		maxNoNew        = 2
+		maxFails        = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		MinFound:     minServers,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: betweenDuration,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+		},
+	}
+	return presolver.Resolve(ctx, hosts, settings)
+}