v0.35.0 — keep the Docker Hub page in step with the GitHub one

dockerhub_private (default false) is read back after every push and corrected
when it differs. Pushing to a repository that does not exist creates it with
whatever visibility the account default says, so the result used to depend on a
setting nobody looks at -- and a private one still gave a green run with a
successful push nobody else could pull.

sync_description sets the Docker Hub short description from the GitHub
repository description, cut to Docker Hub's 100-character cap and counted in
codepoints rather than bytes.

readme_url_header prepends a source link, and the project page when one is set,
to the long description -- Docker Hub has no field for either.

Topics are not synced: Docker Hub's categories are a fixed taxonomy rather than
free-form tags.