chore: run container as non-root user and add healthcheck

[RinZ27]

I've split out the Docker hardening changes as discussed in #2933.

This PR updates the Dockerfile to:

1. Create and switch to a non-root smithy user. I also made sure to chown the /smithy directory so the new user has the right permissions to run the CLI and manage its class data sharing archive.
2. Add a HEALTHCHECK using smithy --version to monitor container health.

I think this covers what we talked about for the container side of things. Let me know if you want any further tweaks to the setup!

[github-actions]

This pull request does not contain a staged changelog entry. To create one, use the ./.changes/new-change command. For example:

./.changes/new-change --pull-requests "#2934" --type feature --description "chore: run container as non-root user and add healthcheck"

Make sure that the description is appropriate for a changelog entry and that the proper feature type is used. See ./.changes/README or run ./.changes/new-change -h for more information.

[RinZ27]

@mtdowling That's a fair point. Since this is primarily a short-lived CLI tool rather than a long-running service, the healthcheck is indeed redundant overhead. I'll go ahead and remove it.

[JordonPhillips]

The health check still seems to be there

[RinZ27]

My apologies for the oversight. I've updated the Dockerfile to remove the HEALTHCHECK completely, also took the opportunity to add the missing changelog entry and updated the commit message to reflect that the healthcheck is no longer included.

@JordonPhillips