Retrieval of ACME directories at startup sometimes fails with DNS resolution error

[itzg]

This might be specific to my microk8s installation, but wanted to note for investigating later:

Caused by: java.net.UnknownHostException: Failed to resolve 'acme-v02.api.letsencrypt.org' after 4 queries
        at io.netty.resolver.dns.DnsResolveContext.finishResolve(DnsResolveContext.java:1047) ~[netty-resolver-dns-4.1.77.Final.jar:4.1.77.Final]
        at io.netty.resolver.dns.DnsResolveContext.tryToFinishResolve(DnsResolveContext.java:1000) ~[netty-resolver-dns-4.1.77.Final.jar:4.1.77.Final]
        at io.netty.resolver.dns.DnsResolveContext.query(DnsResolveContext.java:418) ~[netty-resolver-dns-4.1.77.Final.jar:4.1.77.Final]
        at io.netty.resolver.dns.DnsResolveContext.tryToFinishResolve(DnsResolveContext.java:971) ~[netty-resolver-dns-4.1.77.Final.jar:4.1.77.Final]
        at io.netty.resolver.dns.DnsResolveContext.access$700(DnsResolveContext.java:66) ~[netty-resolver-dns-4.1.77.Final.jar:4.1.77.Final]
        at io.netty.resolver.dns.DnsResolveContext$2.operationComplete(DnsResolveContext.java:471) ~[netty-resolver-dns-4.1.77.Final.jar:4.1.77.Final]

Might need to asynchronously retrieve directories slightly later after startup.