unbreak OCI builds by adding to .helmignore: img, units-description.md, values.schema.yaml

The merge of !3251 (merged) had the unexpected side effect of breaking OCI builds, because the Helm chart archive becomes too big (because of the image in the img directory), and does not fit anymore in the Kubernetes Secret where Helm stores it (full chart is stored there by Helm to allow rollbacks).

We can see at https://gitlab.com/sylva-projects/sylva-core/container_registry/4061187 that today the sylva-units OCI artifacts grew from 130K ( 0.0.0-git-481648f1) to nearly 700K with 0.0.0-git-30d478c8 (this is the compressed size).

The result was failures like this one:

Following resources were not ready when exit condition matched:
    * HelmRelease/sylva-units: InstallFailed - Helm install failed for release sylva-units-preview/sylva-units with chart sylva-units@0.0.0-git-e5d1a70d+1: create: failed to create: 
    Secret "sh.helm.release.v1.sylva-units.v1" is invalid: data: 
    Too long: must have at most 1048576 bytes

This MR adds the problematic img directory to .helmignore along with a few unnecessary files.

After this MR the compressed OCI artifact size is back to ~123K (on 0.0.0-git-2200612f which was produced by this MR).