move reference from gcr to ghcr (#3897)

Signed-off-by: Bob Callaway <[email protected]>

README.md

@@ -63,10 +63,10 @@ introduce breaking changes may be accepted, but will be considered lower priorit
 
 ## Dockerfile
 
-Here is how to install and use cosign inside a Dockerfile through the gcr.io/projectsigstore/cosign image:
+Here is how to install and use cosign inside a Dockerfile through the ghcr.io/sigstore/cosign/cosign image:
 
 ```shell
-FROM gcr.io/projectsigstore/cosign:v1.13.0 as cosign-bin
+FROM gcr.io/sigstore/cosign/cosign:v2.4.1 as cosign-bin
 
 # Source: https://github.com/chainguard-images/static
 FROM cgr.dev/chainguard/static:latest