title: Intermittent connection refused error seemingly related to `localhost` being resolved to IPv6
body: <p>I am not very confident if this issue is related to <code>r2dbc-postgresql</code> but now it only happens to us in R2DBC connection, not with Liquibase which uses JDBC...</p>
  <p>We have 3 integration tests for our tiny project and they use Testcontainer to run a PostgreSQL to support the Repository.
  When building the project on our company cluster, we notice that <em>occasionally</em> some of the tests fail because R2DBC connection to <code>localhost/0:0:0:0:0:0:0:1:&lt;container_exposed_port&gt;</code> is refused.
  Note that all 3 tests depend on DB access and when we repeat the build, each of them has failed in some runs, i.e., in some builds, all 3 tests pass; in some builds, only test 1 fails but 2 and 3 pass; in some builds, only test 2 fails but 1 and 3 pass; in some builds, tests 1 and 2 fail but 3 passes, etc. When one test fails, other tests still run and may pass.</p>
  <p>We've done some debugging and find in <code>netstat -a -n --tcp</code> is suggesting that only <code>0.0.0.0:&lt;container_exposed_port&gt;</code> is being listened on, but not <code>:::&lt;container_exposed_port&gt;</code>.</p>
  <p>After setting system properties for the integration tests to prefer IPv4 stack, the issue goes away, i.e.,</p>
  <pre><code>task integrationTest(type: Test) {
      // ...
      systemProperties = ['java.net.preferIPv4Stack': true] as Map&lt;String, Object&gt;
  }
  </code></pre>
  <p>What I am confused about here is, why when the IPv4 preference is not set, only <em>some</em> of the tests fail. Also, why it does not happen to JDBC connections.</p>