Merge `OTEL_RESOURCE_ATTRIBUTES` when there is one already in the
environment. This allows user-specified resource attributes to be passed
on to CLI plugins while still allowing the extra attributes added for
telemetry information.
This was the original intended use-case but it seems to have never made
it in. The reason `OTEL_RESOURCE_ATTRIBUTES` was used is because we
could combine it with user-centric ones.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
