To obtain root privileges inside an already running Docker container, the `docker exec` command is indeed the most straightforward method. You can try executing the command as follows: docker exec -u 0 -it /bin/bash. This attempts to run a bash shell as the root user. However, if the container is running with security settings that restrict access, such as those found in an unprivileged user context, you might face permission issues. In those cases, it is essential to ensure that your user on the host has the necessary privileges to execute commands inside the container, especially if you have started the container with a non-root user intentionally for security reasons. If the `exec` command fails because of user restrictions, you will need to investigate ways to modify the container’s user settings in your Dockerfile or look into capabilities settings that may allow escalation of privileges without significant compromise of your security posture.

Another approach is to connect to the Docker daemon directly from the host to manipulate the container. This can be achieved using the docker commit command to create a new image from the running container and then run a new container with elevated privileges using that image. Alternatively, if rejoining the original process is your goal, you can explore the use of techniques like attaching to the process groups. Always validate your modifications against best security practices. It’s wise not to modify the original Dockerfile for short-term solutions as it might lead to broader security vulnerabilities. The best practice involves operating under the principle of least privilege while ensuring your development and production environments maintain consistent configurations. Hence, consider using tools like Docker Compose or Kubernetes, which can manage user permissions more granularly. Additionally, always back up your data before making significant changes.