mender-convert: Inside container

mender-convert has a lot of dependencies that has different package names and versions across Linux distributions. Providing a standard way to run mender-convert inside a container would greatly simply installation.

Note that container support is already working for Raspbian-based images (manually tested end result), but not for Debian based images (mount error).

User value (why)

• Faster and easier to get started using mender-convert (installation/setup time)
• No need to "clutter" OS with packages that are only used for running mender-convert
• Support running mender-convert on Mac and other OSes than Linux

Size (SP)

• TBD

Acceptance criteria

• Current docker-mender-convert approach works for all supported image types
• Appropriate tests for using mender-convert in a container created
• Official documentation changed to run inside as container by default
• (stretch goal) mender-convert image uploaded to Docker Hub and released as part of mender-convert releases (new version of mender-convert and Mender client inside)