Move to using ubuntu as base with feature already installed for devcontainer

At the moment we:
- Use `python:3.xx` as the base container
- Make a venv
- Install into it and use that
- Apply the `bash-config` feature on top

This means that vscode is taking up to 30s adding in the feature every time. We were thinking of putting the feature in the base container for `epics-containers` but publishing this for multiple python bases would be prohibitive for the copier template.

However with `uv` we now have a different option:
- Publish an ubuntu LTS derived devcontainer with uv, bash and zsh, and the `bash-config` feature installed
- Use it for both `python-copier-template` and `epics-containers`
- Get `uv` to manage the version of python

This should allow us to have a much better out of the box experience by pulling a single rarely changing base image that will be used for many purposes. The only downside is that every `devcontainer.json` will need to provide the mount of `${localEnv:HOME}/.config/bash-config` to do persistent history, but as we use copier that is easy to arrange for.

As discussed with @gilesknap @callumforrester

Review devcontainer.json and the existing base-container setup, then trace how python-copier-template and epics-containers consume it. Confirm the Ubuntu LTS image includes uv, bash, zsh, and the bash-config feature, and verify that the generated configuration provides the persistent bash-config mount.

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