Separate out XIOS Docker build

Fixes #625

Task List

[x] Defined the tests that specify a complete and functioning change (It may help to create a design specification & test specification)
[x] Implemented the source code change that satisfies the tests
[x] Documented the feature by providing worked example
[x] Updated the README or other documentation
[x] Completed the pre-Request checklist below

Change Description

Currently, we have a Dockerfile.devenv and a Dockerfile.production, which depends on it. The former sets up the build dependencies using Spack, as well as installing XIOS. This is inconvenient for updating XIOS because rebuilding the dev-env image requires rebuilding the Spack environment, too.

This PR separates out Dockerfile.devenv into two steps: Dockerfile.base, which installs the dependencies, and Dockerfile.devenv, which pulls in those and installs XIOS.

I've built the images locally, as well as Dockerfile.production, which is built on top of Dockerfile.devenv. Running the production Docker image built with mpi=ON and xios=ON, I was able to run the tests successfully. Once the reviewer is happy with the changes, I can push the Docker images to ghcr.

Pre-Request Checklist

[x] The requirements of this pull request are fully captured in an issue or design specification and are linked and summarised in the description of this PR
[x] No new warnings are generated
[x] The documentation has been updated (or an issue has been created to track the corresponding change)
[x] Methods and Tests are commented such that they can be understood without having to obtain additional context
[x] This PR/Issue is labelled as a bug/feature/enhancement/breaking change
[x] File dates have been updated to reflect modification date
[x] This change conforms to the conventions described in the README

nextsimhub / nextsimdg