ClassPath resource can not be accessed in Docker Spring - spring

Question

I have a userService file which includes this code:
package com.example.demo.services;
import com.example.demo.entity.User;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.core.io.ClassPathResource;
import org.springframework.stereotype.Service;
import java.io.IOException;
import java.util.List;
import java.util.concurrent.atomic.AtomicReference;
#Service
public class UserService {
public static AtomicReference<UserService> INSTANCE = new AtomicReference<UserService>();
public static List<User> userList ;
public UserService() throws IOException {
final UserService previous = INSTANCE.getAndSet(this);
userList = new ObjectMapper().readValue(
new ClassPathResource("db/user.json").getFile(),
new ObjectMapper().getTypeFactory().constructCollectionType(List.class, User.class));
if(previous != null)
throw new IllegalStateException("Second singleton " + this + " created after " + previous);
}
public static UserService getInstance() {
return INSTANCE.get();
}
}
In this Service I am loading a JSON file into the variable userList and the JSON file is stored in src/main/resources/db/user.json. This works fine in the IDE. But when I creates a Jar file either via Intellij or manually, the db/user.json is stored in BOOT-INF/classes/db/user.json (inspected via this command: jar tf backend.jar).
The Docker Image could not get started as it could not find the file. So how would I change this so that it should work both in normal debug and Docker image?
To be noted, userList converts the JSON file in the List.