The Invoice Ninja container is not designed to be immutable at all; it
makes a bunch of changes to its own contents when it starts up.
Notably, it copies the contents of the `public` and `storage`
directories from the container image to the persistent volume _and then
deletes the source_. Additionally, being a Laravel application, it
needs write access to its own code for caching, etc. Previously, the
`init.sh` script copied the entire `app` directory to a temporary
directory, and then the runtime container mounted that volume over the
top of the original location. This allowed the root filesystem of the
container to be read-only, while the `app` directory was still mutable.
Unfortunately, this makes the startup process incredibly slow, as it
takes a couple of minutes to copy the whole application. It's also
pretty pointless, because the application runs as an unprivileged
process, so it wouldn't have write access to the rest of the filesystem
anyway. As such, I've decided to remove the `readOnlyRootFilesytem`
restriction, and allow the container to run as upstream intends, albeit
begrudgingly.
Dustin