Cache images by SHA

**What is the problem you're trying to solve**

To improve the security posture of our supply chain, we wish to reference container images by SHA rather than tags, which are mutable.

**Describe the solution you'd like**

I would like to reference images by SHA and have ACR retrieve the appropriate SHA of the upstream image.

**Additional context**

Error message:

```
#2 [auth] actions/actions-runner:pull token for ORG_REGISTRY.azurecr.io
#2 DONE 0.0s

#3 [internal] load metadata for ORG_REGISTRY.azurecr.io/actions/actions-runner:sha256-831a2607a2618e4b79d9323b4c72330f3861768a061c2b92a845e9d214d80e5b
#3 ERROR: encountered unknown type application/vnd.oci.empty.v1+json; children may not be fetched
------
> [internal] load metadata for ORG_REGISTRY.azurecr.io/actions/actions-runner:sha256-831a2607a2618e4b79d9323b4c72330f3861768a061c2b92a845e9d214d80e5b:
------
Dockerfile:1
--------------------
1 | >>> FROM ORG_REGISTRY.azurecr.io/actions/actions-runner:sha256-831a2607a2618e4b79d9323b4c72330f3861768a061c2b92a845e9d214d80e5b
2 | # sha256-831a2607a2618e4b79d9323b4c72330f3861768a061c2b92a845e9d214d80e5b = 2.323.0
3 | # see https://github.com/actions/runner/pkgs/container/actions-runner for versions
--------------------
ERROR: failed to solve: ORG_REGISTRY.azurecr.io/actions/actions-runner:sha256-831a2607a2618e4b79d9323b4c72330f3861768a061c2b92a845e9d214d80e5b: failed to resolve source metadata for ORG_REGISTRY.azurecr.io/actions/actions-runner:sha256-831a2607a2618e4b79d9323b4c72330f3861768a061c2b92a845e9d214d80e5b: encountered unknown type application/vnd.oci.empty.v1+json; children may not be fetched
```