VPN Unlimited support (#499)

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

internal/updater/providers/vpnunlimited/servers.go

@@ -0,0 +1,42 @@
+// Package vpnunlimited contains code to obtain the server information
+// for the VPNUnlimited provider.
+package vpnunlimited
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
+	"github.com/qdm12/gluetun/internal/updater/unzip"
+)
+
+var ErrNotEnoughServers = errors.New("not enough servers found")
+
+func GetServers(ctx context.Context, unzipper unzip.Unzipper,
+	presolver resolver.Parallel, minServers int) (
+	servers []models.VPNUnlimitedServer, warnings []string, err error) {
+	// Hardcoded data from a user provided ZIP file since it's behind a login wall
+	hts, warnings := getHostToServer()
+
+	hosts := hts.toHostsSlice()
+	hostToIPs, newWarnings, err := resolveHosts(ctx, presolver, hosts, minServers)
+	warnings = append(warnings, newWarnings...)
+	if err != nil {
+		return nil, warnings, err
+	}
+
+	hts.adaptWithIPs(hostToIPs)
+
+	servers = hts.toServersSlice()
+
+	if len(servers) < minServers {
+		return nil, warnings, fmt.Errorf("%w: %d and expected at least %d",
+			ErrNotEnoughServers, len(servers), minServers)
+	}
+
+	sortServers(servers)
+
+	return servers, warnings, nil
+}