Add ability to use find and replace interpolation

It would be very helpful to be able to do simple find and replace with interpolation. My use case, for example, would be to use the Git branch name as a Docker image tag but to do so I need to replace certain characters that are valid in Git but not in Docker — e.g., replace forward slashes with dashes.

It would be great to be able to do something like this:

```
- label: "Push images tagged with Git branch"
plugins:
- docker-compose#v3.7.0:
config: buildkite/docker/compose.yml
push:
- foo:123456789.dkr.ecr.eu-west-2.amazonaws.com/foo:${BUILDKITE_BRANCH//\\//--}
```

There you can see the desired interpolation that converts a `/` character to `--`

I'm afraid I don't know Go so can't offer a PR.