Docker doesn't do mods. As the article says, this is possible due to s6 and s6-overlay, which is included with linuxserver.io docker images, combined with a set

Docker doesn't do mods. As the article says, this is possible due to s6 and s6-overlay, which is included with linuxserver.io docker images, combined with a set up scripts that set it all up. This does prevent your containers from being immutable. All the code for LSIO images is available on their GitHub.