Use the pg_isready command to asses wether a PostgreSQL is ready or not

See docker-library/postgres#146 (comment)

src/Testcontainers.PostgreSql/PostgreSqlBuilder.cs

@@ -123,18 +123,10 @@ protected override PostgreSqlBuilder Merge(PostgreSqlConfiguration oldValue, Pos
     /// <inheritdoc cref="IWaitUntil" />
     private sealed class WaitUntil : IWaitUntil
     {
-        private static readonly string[] LineEndings = { "\r\n", "\n" };
-
         /// <inheritdoc />
-        public async Task<bool> UntilAsync(IContainer container)
+        public Task<bool> UntilAsync(IContainer container)
         {
-            var (stdout, stderr) = await container.GetLogsAsync(timestampsEnabled: false)
-                .ConfigureAwait(false);
-
-            return 2.Equals(Array.Empty<string>()
-                .Concat(stdout.Split(LineEndings, StringSplitOptions.RemoveEmptyEntries))
-                .Concat(stderr.Split(LineEndings, StringSplitOptions.RemoveEmptyEntries))
-                .Count(line => line.Contains("database system is ready to accept connections")));
+            return ((PostgreSqlContainer)container).IsReadyAsync();
         }
     }
 }

src/Testcontainers.PostgreSql/PostgreSqlContainer.cs

@@ -48,4 +48,19 @@ await CopyAsync(Encoding.Default.GetBytes(scriptContent), scriptFilePath, Unix.F
         return await ExecAsync(new[] { "psql", "--username", _configuration.Username, "--dbname", _configuration.Database, "--file", scriptFilePath }, ct)
             .ConfigureAwait(false);
     }
+
+    /// <summary>
+    /// Test whether the database is ready to accept connections or not with the <c>pg_isready</c> command.
+    /// </summary>
+    /// <returns><see langword="true"/> if the database is ready to accept connections; <see langword="false"/> if the database is not yet ready.</returns>
+    internal async Task<bool> IsReadyAsync()
+    {
+        var command = new[] {
+            "pg_isready",
+            "--dbname", _configuration.Database,
+            "--username", _configuration.Username,
+        };
+        var result = await ExecAsync(command);
+        return result.ExitCode == 0;
+    }
 }