Cache image prior to checking docker user mapping

[rmartin16]

Changes

• Closes Better user feedback when Linux system package target is not available in the Docker registry #1368
• Simply ensuring the image is cached beforehand prints out a much better error message.
• While the docker return code checking may prove useful in the future, it feels like overkill right now.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Looks like a good tweak to me; I've made one minor update adding some cosmetic logging. Turning off log streaming for the download is a good improvement, but it means the first line of output is Docker download details, rather than Briefcase telling you what it is doing.