Use the UM2N Docker image

[jwallwork23]

Closes #57.

Linked PR: mesh-adaptation/docs#59.

This PR sets up UM2N's CI to use the newly added bespoke Docker image. It also makes use of the reusable test workflow from the docs repo.

[jwallwork23]

This seems to work, although it's worth noting that there CI does raise an (inconsequential) error when pip installing requirements that are already installed (see https://github.com/mesh-adaptation/UM2N/actions/runs/11899297143/job/33157629345?pr=59):

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
cffconvert 2.0.0 requires jsonschema<4,>=3.0, but you have jsonschema 4.23.0 which is incompatible.
Successfully installed UM2N-1.0 aiohappyeyeballs-2.4.3 aiohttp-3.11.2 aiosignal-1.3.1 argon2-cffi-23.1.0 argon2-cffi-bindings-21.2.0 async-lru-2.0.4 babel-2.16.0 beautifulsoup4-4.12.3 bleach-6.2.0 defusedxml-0.7.1 einops-0.8.0 fqdn-1.5.1 frozenlist-1.5.0 fsspec-2024.10.0 gmsh-4.13.1 ipywidgets-8.1.5 isoduration-20.11.0 json5-0.9.28 jsonpointer-3.0.0 jsonschema-4.23.0 jupyter-1.1.1 jupyter-console-6.6.3 jupyter-events-0.10.0 jupyter-lsp-2.2.5 jupyter-server-2.14.2 jupyter-server-terminals-0.5.3 jupyterlab-4.2.6 jupyterlab-pygments-0.3.0 jupyterlab-server-2.27.3 jupyterlab-widgets-3.0.13 meshio-5.3.5 mistune-3.0.2 multidict-6.1.0 nbclient-0.10.0 nbconvert-7.16.4 networkx-3.4.2 notebook-7.2.2 notebook-shim-0.2.4 nvidia-cublas-cu12-12.4.5.8 nvidia-cuda-cupti-cu12-12.4.127 nvidia-cuda-nvrtc-cu12-12.4.127 nvidia-cuda-runtime-cu12-12.4.127 nvidia-cudnn-cu12-9.1.0.70 nvidia-cufft-cu12-11.2.1.3 nvidia-curand-cu12-10.3.5.147 nvidia-cusolver-cu12-11.6.1.9 nvidia-cusparse-cu12-12.3.1.170 nvidia-nccl-cu12-2.21.5 nvidia-nv

Edit: the error isn't actually so inconsequential.

[jwallwork23]

Just realised this is flawed - it's using the firedrake-parmmg image rather than the firedrake-um2n one. Need to add an option in the reusable workflow to switch between them.

[jwallwork23]

Force pushed without the branch-specific commit now that mesh-adaptation/docs#67 has been merged. Should be all good now I think.