cert-manager: Add DNS.01 solver using Cloudflare

Using *acme-dns.io* is incredibly cumbersome.  Since each unique
subdomain requires its own set of credentials, the `acme-dns.json` file
has to be updated every time a new certificate is added.  This
effectively precludes creating certificates via Ingress annotations.

As Cloudflare's DNS service is free and anonymous as well, I thought I
would try it out as an alternative to *acme-dns.io*.  It seems to work
well so far.  One potential issue, though, is Cloudflare seems to have
several nameservers, with multiple IP addresses each.  This may require
adding quite a few exceptions to the no-outbound-DNS rule on the
firewall.  I tried using the "recursive servers only" mode of
*cert-manager*, however, as expected, the recursive servers all cache
too aggressively.  Since the negative cache TTL value in the SOA record
for Cloudflare DNS zones is set to 1 hour and cannot be configured, ACME
challenges can take at least that long in this mode.  Thus, querying the
authoritative servers directly is indeed the best option, even though it
violates the no-outbound-DNS rule.

cert-manager/.gitignore

@@ -1,4 +1,5 @@
 acme-dns.json
 cert-exporter.pem
 cert-manager.key
+cloudflare.api-token
 zerossl.secret

cert-manager/cluster-issuer.yaml

@@ -15,6 +15,12 @@ spec:
       name: zerossl-prod
 
     solvers:
+    - dns01:
+        cnameStrategy: Follow
+        cloudflare:
+          apiTokenSecretRef:
+            name: cloudflare
+            key: cloudflare.api-token
     - dns01:
         cnameStrategy: Follow
         acmeDNS:
@@ -22,3 +28,14 @@ spec:
           accountSecretRef:
             name: acme-dns
             key: acme-dns.json
+      selector:
+        dnsNames:
+        - pyrocufflink.blue
+        - '*.pyrocufflink.blue'
+        - pyrocufflink.net
+        - '*.pyrocufflink.net'
+        - dustin.hatch.name
+        - '*.dustin.hatch.name'
+        - aimee-os.org
+        - '*.aimee-os.org'
+        - '*.import.firefly.pyrocufflink.blue'

cert-manager/kustomization.yaml

@@ -35,3 +35,10 @@ secretGenerator:
   - acme-dns.json
   options:
     disableNameSuffixHash: true
+
+- name: cloudflare
+  namespace: cert-manager
+  files:
+  - cloudflare.api-token
+  options:
+    disableNameSuffixHash: true