feature: add exist_version tag

While exist is not included in the image, it makes for more maintainable Dockerfiles, ala:

```Dockerfile
ARG EXIST_VERSION=6.2.0
FROM ghcr.io/eeditiones/builder:${EXIST_VERSION} AS builder
# do your thing
FROM ghcr.io/jinntec/base:${EXIST_VERSION}
# more stuff
```

alternatively only add `:release` as tag… tbd