Fix Jaeger Tracing in Docker Compose

[abeatrix]

Jaeger is not working on Docker Compose currently due to missing env var stated in https://github.com/sourcegraph/sourcegraph/issues/21677#issuecomment-889433798
This PR adds the missing env var SAMPLING_STRATEGIES_FILE=/etc/jaeger/sampling_strategies.json so that jaeger can work on Docker Compose instances properly.

Checklist

• Sister deploy-sourcegraph change:
• All images have a valid tag and SHA256 sum

[bobheadxi]

Seemed to pass after giving the CI a kick, seemed to be a network flake :)

[abeatrix]

Thanks for checking @bobheadxi ! Will I need to update the changelog to reflect the changes as well?

[bobheadxi]

I would recommend something like this:

Changed

• docker-compose: the Jaeger container's SAMPLING_STRATEGIES_FILE now has a default value. If you are currently using a custom sampling strategies configuration, you may need to make sure your configuration is not overridden by the change when upgrading.