Start milestone 3

- Add functionality to query online sources (Keyservers, etc) for
  certificates.
- Use introducers to determine which UserIDs should be fetched from
  online soources. Check certifications before importing certifiactes
  into the local cert store.
- start docker image for local end-to-end testing.

src/types/introducer.rs

@@ -29,5 +29,29 @@ impl Introducer {
             false
         }
     }
+
+    pub fn is_introducing(&self, cert: &Cert) -> bool {
+
+        let policy = StandardPolicy::new();
+        let mut result = false;
+
+        let my_keyid = self.cert.keyid();
+
+        if let Ok(vc) = cert.with_policy(&policy, None) {
+            vc.userids()
+                .revoked(false)
+                .for_each(|u| {
+                    u.certifications().for_each(|s| {
+                        if s.signature_alive(None, None).is_ok() {
+                            s.issuers().for_each(|i| {
+                                if i == &my_keyid { result = true; }          
+                            });
+                        }
+                    })
+                });
+        }
+
+        result
+    }
 }