What does the `--read-only` flag do in Docker?

The --read-only flag in Docker is used to set the container's root filesystem to read-only. This means that once the container is started with this flag, the processes running in the container will not be able to modify files in the root filesystem. This is particularly useful in scenarios where you want to ensure that the integrity of the application and its running environment is maintained, preventing any unintended changes or corruption of files.

By using this feature, you can enhance security, as it limits the impact of any modifications that a potentially compromised application could make to the filesystem. It also helps in scenarios where the application does not need to write data to the filesystem, focusing instead on improving performance and reliability.

In contrast, the other options suggest capabilities that are contrary to what the --read-only flag provides, such as allowing writing or making the filesystem mutable, which would enable changes to be made, undermining the purpose of the read-only setting.