ci: use sha256 method for specifying testrunner in docker-compose.yml [backport 2.5] (#8518)

Backport b809e38 from #8451 to 2.5.

The images specified in `.circleci/config.templ.yaml` and in
`docker-compose.yaml` are currently the same (since #7785) :


https://github.com/DataDog/dd-trace-py/blob/main/.circleci/config.templ.yml#L7C39-L7C149
```
ghcr.io/datadog/dd-trace-py/testrunner@sha256:e1331dde65a56e8c6dc0516c84ef3525602bb36347d28526837e4bf21cdbf1a2
```
and
https://github.com/DataDog/dd-trace-py/blob/main/docker-compose.yml#L157C9-L157C95
```
image: ghcr.io/datadog/dd-trace-py/testrunner:253febbef2d235970f1c71ced77df6fe4fbeb449
```

and `docker inspect` to the same thing: 
```
% docker inspect ghcr.io/datadog/dd-trace-py/testrunner@sha256:e1331dde65a56e8c6dc0516c84ef3525602bb36347d28526837e4bf21cdbf1a2
[
    {
        "Id": "sha256:bd91eee15360327ed48119f03111d0d826bff23043fd2fb34fe30b1623a30bf8",
        "RepoTags": [
            "ghcr.io/datadog/dd-trace-py/testrunner:253febbef2d235970f1c71ced77df6fe4fbeb449",
            "ghcr.io/datadog/dd-trace-py/testrunner:latest"
        ],
        "RepoDigests": [
            "ghcr.io/datadog/dd-trace-py/testrunner@sha256:e1331dde65a56e8c6dc0516c84ef3525602bb36347d28526837e4bf21cdbf1a2"
        ],
```
vs
```
% docker inspect ghcr.io/datadog/dd-trace-py/testrunner:253febbef2d235970f1c71ced77df6fe4fbeb449
[
    {
        "Id": "sha256:bd91eee15360327ed48119f03111d0d826bff23043fd2fb34fe30b1623a30bf8",
        "RepoTags": [
            "ghcr.io/datadog/dd-trace-py/testrunner:253febbef2d235970f1c71ced77df6fe4fbeb449",
            "ghcr.io/datadog/dd-trace-py/testrunner:latest"
        ],
        "RepoDigests": [
            "ghcr.io/datadog/dd-trace-py/testrunner@sha256:e1331dde65a56e8c6dc0516c84ef3525602bb36347d28526837e4bf21cdbf1a2"
        ],
```

This change switches the `docker-compose.yml` to use the same way of
specifying the image so that it's easier to see if the images are in
sync in the future.

## Checklist

- [x] Change(s) are motivated and described in the PR description
- [x] Testing strategy is described if automated tests are not included
in the PR
- [x] Risks are described (performance impact, potential for breakage,
maintainability)
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed or label `changelog/no-changelog` is set
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/))
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))
- [x] If this PR changes the public interface, I've notified
`@DataDog/apm-tees`.
- [x] If change touches code that signs or publishes builds or packages,
or handles credentials of any kind, I've requested a review from
`@DataDog/security-design-and-guidance`.

## Reviewer Checklist

- [x] Title is accurate
- [x] All changes are related to the pull request's stated goal
- [x] Description motivates each change
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- [x] Testing strategy adequately addresses listed risks
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] Release note makes sense to a user of the library
- [x] Author has acknowledged and discussed the performance implications
of this PR as reported in the benchmarks PR comment
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Romain Komorn <[email protected]>

Author: github-actions[bot]

docker-compose.yml

@@ -154,7 +154,7 @@ services:
           - "127.0.0.1:5433:5433"
 
     testrunner:
-        image: ghcr.io/datadog/dd-trace-py/testrunner:253febbef2d235970f1c71ced77df6fe4fbeb449
+        image: ghcr.io/datadog/dd-trace-py/testrunner@sha256:e1331dde65a56e8c6dc0516c84ef3525602bb36347d28526837e4bf21cdbf1a2
         command: bash
         environment:
             - TOX_SKIP_DIST=True