[Bug][Identity Server] Configuration is not always correctly loaded

[pm-McFly]

During CI tests, https://github.com/linagora/ToM-server/blob/compose-update/packages/federated-identity-service/src/__testData__/identity-server/Dockerfile is using this server.mjs:
https://github.com/linagora/ToM-server/blob/compose-update/packages/federated-identity-service/src/__testData__/identity-server/server.mjs

If the same Dockerfile uses the "official" server.mjs, the configuration is wrong:
https://github.com/linagora/ToM-server/blob/compose-update/packages/matrix-identity-server/server.mjs

Expect:
expected.json

Got:
got.json