Allows running a vendored version of Coder in our `compose.yaml` as
follows:
```
CODER_REPO=my.repo.tld/mycoder/coder CODER_VERSION=my-tag docker compose up
```
Also allows running the current "dogfood" version of Coder as follows:
```
CODER_REPO=ghcr.io/coder/coder-preview CODER_VERSION=dogfood docker compose up
```