Update to newer Docker base image to have access to newer software versions

[Leonidas-from-XIV]

The Dockerfile used opam:alpine-3.19-ocaml as a build image, which was created before the opam-repo archival and the opam update didn't do anything as it would only consult the local checkout of opam-repository (/home/opam/opam-repository), which did not update anything.

This means that e.g. the archived versions of packages are still available and packages are stuck at old versions (e.g. Dune 3.16 released in June 2024).

This PR

• updates the image to a newer version of Alpine Linux (3.19 is EOL and while it can be made work, might as well switch to a currently supported version since the only change required is the version in the FROM line)
• Uses opam 2.5, since the image supports it, might as well. This incorporates depext directly, so the separate call was removed
• Adds the archive repo to include the archived versions of the dependencies
• Updates both the regular Dockerfile which generates an executable as well as the Dockerfile.test which generates a build log. The build log now contains both unit and E2E test results as we're running dune build only once (dune build creates a new _build/log on every run, so dune build @e2e would overwrite the log of dune runtest).

This should make the setup in the Docker container a bit more futureproof with never versions of software as well as being closer to the set up people need to develop locally (especially the archive repository).

cc @djs55