Revert "Always set the ORACLE_DATABASE environment variable"

This reverts commit e17bd73.

Container would fail to start with this error.
> ORA-65012: Pluggable database FREEPDB1 already exists.

src/Testcontainers.Oracle/OracleBuilder.cs

@@ -47,8 +47,7 @@ private OracleBuilder(OracleConfiguration resourceConfiguration)
     /// <returns>A configured instance of <see cref="OracleBuilder" />.</returns>
     public OracleBuilder WithDatabase(string database)
     {
-        return Merge(DockerResourceConfiguration, new OracleConfiguration(database: database))
-            .WithEnvironment("ORACLE_DATABASE", database);
+        return Merge(DockerResourceConfiguration, new OracleConfiguration(database: database));
     }
 
     /// <summary>
@@ -85,6 +84,11 @@ public override OracleContainer Build()
             return new OracleContainer(WithDatabase(defaultServiceName).DockerResourceConfiguration);
         }
 
+        if (DockerResourceConfiguration.Database != defaultServiceName)
+        {
+            return new OracleContainer(WithEnvironment("ORACLE_DATABASE", DockerResourceConfiguration.Database).DockerResourceConfiguration);
+        }
+
         return new OracleContainer(DockerResourceConfiguration);
     }