Add docker image signature with cosign

[acouvreur]

Docker image should be signed with cosign.

See https://github.com/sablierapp/mimic goreleaser configuration. Everything is sign "keyless" using OIDC token from github.

Documentation should be updated so users know how to ensure where the image is coming from.

Signature does not mean safe, it means that it states who it is and where it comes from.

As a user, you should be careful with projects such as Sablier which needs access control over sockets or orchestrators.