Refine CI docker cache handling, and obtain 100% cache hit during containers building.

While playing with docker bake action and github action based docker build layer cache, i've struggled a bit to discover that, in case of a multi target baking, every target must have its own scope (see: docker/bake-action#91 (comment)).

As this repo uses 2 targets, i've just used them as a prefix for each target cache scope:

$ docker buildx bake -f compose.yaml -f compose.override.yaml --print
...
{
  "group": {
    "default": {
      "targets": [
        "php",
        "pwa"
      ]
    }
  },
...