@Contract("_, !null -> !null")
    public String getEnvVarOrProperty(@NotNull final String propertyName, @Nullable final String defaultValue) {
        return getConfigurable(propertyName, defaultValue, userProperties, classpathProperties);
    }
Gets a configured setting from an environment variable (if present) or a configuration file property otherwise.
The configuration file will be the <code>.testcontainers.properties</code> file in the user's home directory or
a <code>testcontainers.properties</code> found on the classpath.
<p>
Note that when searching environment variables, the prefix `TESTCONTAINERS_` will usually be applied to the
property name, which will be converted to upper-case with underscore separators. This prefix will not be added
if the property name begins `docker.`.

@param propertyName name of configuration file property (dot-separated lower case)
@return the found value, or null if not set
core/src/main/java/org/testcontainers/utility/TestcontainersConfiguration.java