Changing LocalStack Account ID to a Custom Value (not defualt 000000000000)

Question

I'm currently working with localstack on creating some integration tests.

I'd like to configure the account ID to something other then the default 000000000000

I found this closed bug which implies that one can set the account id via the environemnt variable TEST_AWS_ACCOUNT_ID. At least that's how I understand it.

Unfortunately when I set this, and then run the following code in a test

val stsClient: AWSSecurityTokenService =
  AWSSecurityTokenServiceClientBuilder.standard()
    .withCredentials(credentialsProviders.v1)
    .withEndpointConfiguration(new EndpointConfiguration(localStack.endpointOverride(LocalStackContainer.Service.STS).toString, v1Region.value.getName))
    .build()

val thisAccountId = AccountId(stsClient.getCallerIdentity(new GetCallerIdentityRequest()).getAccount)
println(s"ThisAccountID: $thisAccountId")

I get the following printout ThisAccountID: 000000000000

I'd greatly appreciate any insights into whether it's possible to change the account ID and, if so, how to achieve this. Thank you for your help