`--set '.base = ...'` fails with "field `base` must be empty for YAML validation"

[jkylling]

Description

Using --set '.base = ...' fails with limactl create, limactl start, and limactl clone.

# Create a simple test template
cat > /tmp/test.yaml << 'EOF'
images:
- location: https://example.com/test.qcow2
  arch: x86_64
EOF

# Test 1: limactl create - FAILS
limactl create --tty=false --name=test --set '.base = ["template://alpine"]' /tmp/test.yaml

# Create instance to start/clone
limactl create --tty=false --name=existing-instance template:default

# Test 2: limactl start on existing instance - FAILS
limactl start --tty=false --set '.base = ["template://alpine"]' existing-instance

# Test 3: limactl clone - FAILS
limactl clone --tty=false --set '.base = ["template://alpine"]' existing-instance cloned-instance

All three commands fail with variations of:

FATA[0000] field `base` must be empty for YAML validation

This might be similar to #3741.

[jandubois]

The root cause is that base is handled in the "template embedding" stage that happens very early, before you can edit the resulting template. Once the template is "fully embedded" the base processing does not happen again.

It is somewhat similar to the fact that you cannot change the template anymore once you have created the instance (similar, but not the same, it is kind of a complex dependency chain).

We can maybe revisit this issue when the FillDefaults() logic is rewritten to also use template embedding instead of doing this in Go code.

Also related is that we have lima.yaml settings that cannot be changed after the instance has been created, but we don't have proper warnings about this. E.g. you can edit the images field all you want, but it has no effect once the instance has been created. And e.g. changing the VM type from QEMU to VZ or vice versa might actually break an existing instance.