Pin redis image version in docker compose file

[Bomme]

Currently we use the redis:alpine image tag which might resolve to a different image (with a "new" redis version on pull)
We should instead use a redis:<version>-alpine tag.
See the list of supported versions/tags

[Bomme]

@ffont could you let me know what version we should pin to match the one currently running in production?

[ffont]

Hi, in production we use an image called registry.sb.upf.edu/mtg/redis:6.2.5 which I suspect is redis 6.2.5, however, for some strange reason I can't find any code that generates this image so at this point I'm not sure why we're using this custom redis thing. To solve this issue you can pin to 6.2.5, but I'm a bit confused now as per why we're not using the standard redis image in production and whether we could switch to it. Do you remember anything about that @alastair?