title: Keycloak Gatekeeper always fail to validate 'iss' claim value
body: <p>Adding the <code>match-claims</code> to the configuration file doesn't seem to do anything. Actually, <strong>Gatekeeper</strong> is always throwing me the same error when opening a resource (with or without the property).</p>
  
  <p>My <strong>Keycloak</strong> server is inside a docker container, accessible from an internal network as <code>http://keycloak:8080</code> while accessible from the external network as <code>http://localhost:8085</code>.</p>
  
  <p>I have <strong>Gatekeeper</strong> connecting to the Keycloak server in an internal network. The request comes from the external one, therefore, the <code>discovery-url</code> will not match the <strong>'iss'</strong> token claim.</p>
  
  <p>Gatekeeper is trying to use the <code>discovery-url</code> as <strong>'iss'</strong> claim. To override this, I'm adding the <code>match-claims</code> property as follows:</p>
  
  <pre><code>discovery-url: http://keycloak:8080/auth/realms/myRealm
  match-claims:
    iss: http://localhost:8085/auth/realms/myRealm
  </code></pre>
  
  <p>The logs look like:</p>
  
  <p><strong>On startup</strong></p>
  
  <pre><code>keycloak-gatekeeper_1  | 1.5749342705316222e+09 info    token must contain
    {"claim": "iss", "value": "http://localhost:8085/auth/realms/myRealm"}
  
  keycloak-gatekeeper_1  | 1.5749342705318246e+09 info    keycloak proxy service starting
    {"interface": ":3000"}
  </code></pre>
  
  <p><strong>On request</strong></p>
  
  <pre><code>keycloak-gatekeeper_1  | 1.5749328645243566e+09 error   access token failed verification
    { "client_ip": "172.22.0.1:38128",
      "error": "oidc: JWT claims invalid: invalid claim value: 'iss'.
             expected=http://keycloak:8080/auth/realms/myRealm,
             found=http://localhost:8085/auth/realms/myRealm."}
  </code></pre>
  
  <p>This ends up in a <strong>403 Forbidden</strong> response.</p>
  
  <hr>
  
  <p>I've tried it on <strong>Keycloak-Gatekeeper</strong> <strong>8.0.0</strong> and <strong>5.0.0</strong>, both with the same issue.</p>
  
  <ol>
  <li>Is this supposed to work the way I'm trying to use it?</li>
  <li>If not, what I'm missing?, how can I validate the <strong>iss</strong> or bypass this validation? (preferably the former)?</li>
  </ol>