fix Docker build in CI; release mockotlpserver 0.6.2

[trentm]

Fix another couple issues with Docker build/push in CI.
This fixes a failure seen here: https://github.com/elastic/elastic-otel-node/actions/runs/12382422883
Also add the requisite token permission for pushing a package to ghcr.io.

[trentm]

THis worked finally:

• GH release: https://github.com/elastic/elastic-otel-node/releases/tag/mockotlpserver-v0.6.2
• Docker image:

% docker buildx imagetools inspect ghcr.io/elastic/elastic-otel-node/mockotlpserver
Name:      ghcr.io/elastic/elastic-otel-node/mockotlpserver:latest
MediaType: application/vnd.oci.image.index.v1+json
Digest:    sha256:0f64f07d0c95562fb3f96905b1efd5561ceaa33dc85a96c27d63d235ad1b4802
...

• npm:

% npm info @elastic/mockotlpserver time
{
  created: '2024-09-06T17:02:49.407Z',
  modified: '2024-12-18T16:16:34.296Z',
  '0.4.1': '2024-09-06T17:02:49.736Z',
  '0.5.0': '2024-09-06T18:34:07.973Z',
  '0.6.2': '2024-12-18T16:16:32.441Z'
}
```