blockcheck: allow to use custom DoH servers

blockcheck.sh

@@ -53,7 +53,7 @@ NFT_TABLE=blockcheck
 
 DNSCHECK_DNS=${DNSCHECK_DNS:-8.8.8.8 1.1.1.1 77.88.8.1}
 DNSCHECK_DOM=${DNSCHECK_DOM:-pornhub.com ntc.party rutracker.org www.torproject.org bbc.com}
-DOH_SERVERS="https://cloudflare-dns.com/dns-query https://dns.google/dns-query https://dns.quad9.net/dns-query https://dns.adguard.com/dns-query https://common.dot.dns.yandex.net/dns-query"
+DOH_SERVERS=${DOH_SERVERS:-"https://cloudflare-dns.com/dns-query https://dns.google/dns-query https://dns.quad9.net/dns-query https://dns.adguard.com/dns-query https://common.dot.dns.yandex.net/dns-query"}
 DNSCHECK_DIG1=/tmp/dig1.txt
 DNSCHECK_DIG2=/tmp/dig2.txt
 DNSCHECK_DIGS=/tmp/digs.txt