Allow custom kernel boot args via --kernel-arg

[arirubinstein]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

What

Adds a repeatable --kernel-arg flag to container run/container create for appending arbitrary boot arguments to the kernel command line.

container run --kernel /path/to/custom-bzImage \
  --kernel-arg lsm=lockdown,capability,landlock,yama,apparmor,bpf \
  ...

Why

The runtime hardcodes lsm=lockdown,capability,landlock,yama,apparmor (and oops=panic) onto every kernel command line in RuntimeService.bootstrap. With a custom kernel there is no way to adjust this — e.g. to enable BPF LSM you need lsm=...,bpf, which is currently impossible. More generally there is no escape hatch for any boot-time kernel argument.

How

• New --kernel-arg <arg> option (repeatable) on Flags.Management.
• Utility.getKernel appends the user args onto kernel.commandLine.kernelArgs, which is persisted into the container bundle.
• RuntimeService.bootstrap now applies its built-in defaults per-key, skipping any default whose key the user already supplied. Defaults are expressed as a small keyed table, so this also lets oops= be overridden and makes future defaults easy to add.

Default behavior is unchanged for anyone who does not pass --kernel-arg — the same oops=panic and lsm=... args are applied.

Testing

• swift build clean.
• container run --help renders the new flag.