CI: add "jesec/rtorrent:latest" tag to rolling release

[DopyConqueror]

Description

Docker's default tag is "latest", so when not specifying a tag when pulling/pushing a tag from/to Docker Hub "latest" is used. The "latest" tag was used for "jesec/rtorrent" previously but is not used anymore, leaving an outdated tag on Docker Hub. This outdated tag gets downloaded by default. This affects some of the docker-compose examples, too.

This PR adds the "latest" tag as a synonym for the "master" tag.

Types of changes

• [ ] Breaking change (changes that break backward compatibility of public API or CLI - semver MAJOR)
• [ ] New feature (non-breaking change which adds functionality - semver MINOR)
• [x] Bug fix (non-breaking change which fixes an issue - semver PATCH)

[jesec]

No. It is intended. latest should point to a released build.