chore: Updates opentelemetry-collector docker image (#1193)
chore: Updates opentelemetry-collector docker image (#1193)

Since 'examples/otel-collector' might be the first contact for ruby developers with `otel-collector`, using the `contrib` docker image version in `OTELCOL_IMG` variable makes easier to explore the example with exporters included only on `contrib` docker image version (eg: datadog). It should not conflict with https://github.com/open-telemetry/opentelemetry-ruby/pull/970 since the image version still pinned.

Co-authored-by: Matthew Wear <e5e1d45a9537da06cde713c2438d8f8cc3e64a67@gmail.com>
open-telemetry/opentelemetry-ruby,open-telemetry/opentelemetry-ruby