Packer's docker plugin metadata change for Env does require equal sign and cannot extend env vars in hcl2 variant language

```
source "docker" "ubi9-minimal" {
image = "proxy-registry.example.com/library/external/registry.access.redhat.com/ubi9-minimal:latest"
commit = true
# https://developer.hashicorp.com/packer/plugins/builders/docker#basic-example-changes-to-metadata
changes = [
"ENV MOO=foo",
"ENV PATH=${local.JDK_DIR}:$PATH"
]
}

```
hcl2

1. Contrary to the documentation Packer v1.8.6 requires the "=" sign to alter the environment variables
2. Cannot extend (inherit and add) existing env vars like PATH

Please fix the doc and add that feature if I am correct.

Thanks!