VPN Unlimited support (#499)

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

internal/updater/providers/vpnunlimited/hosttoserver.go

@@ -0,0 +1,38 @@
+package vpnunlimited
+
+import (
+	"net"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+type hostToServer map[string]models.VPNUnlimitedServer
+
+func (hts hostToServer) toHostsSlice() (hosts []string) {
+	hosts = make([]string, 0, len(hts))
+	for host := range hts {
+		hosts = append(hosts, host)
+	}
+	return hosts
+}
+
+func (hts hostToServer) adaptWithIPs(hostToIPs map[string][]net.IP) {
+	for host, IPs := range hostToIPs {
+		server := hts[host]
+		server.IPs = IPs
+		hts[host] = server
+	}
+	for host, server := range hts {
+		if len(server.IPs) == 0 {
+			delete(hts, host)
+		}
+	}
+}
+
+func (hts hostToServer) toServersSlice() (servers []models.VPNUnlimitedServer) {
+	servers = make([]models.VPNUnlimitedServer, 0, len(hts))
+	for _, server := range hts {
+		servers = append(servers, server)
+	}
+	return servers
+}