Update dependencies in devcontainer Dockerfile

[github-actions]

We have pinned the linux dependencies in the devcontainer Dockerfile, but there is no mechanism to automatically update them, currently. It looks like it's on Dependabot's roadmap, so this GitHub Issue gets automatically created every 6 months to:

• update the dependencies manually:
  1. Temporarily unpin the versions (i.e. remove =<version> from each package in the Dockerfile)
  2. Execute the Dockerfile (e.g. if it's a remote container Dockerfile build the remote container)
  3. Run apt-cache policy <package> for each package, to see the version installed
  4. Pin all the versions, replacing any old versions with new ones
• see if Dependabot now offer this functionality (in which case we can do it automatically, from then on)