LRN-34037 Option 2: Containerise with extra makefile

[david-scarratt-lrn]

Summary

Add the ability to build and test in a container. The container command is in an external makefile, requiring selection using -f docker.mk.

Pros

• Doesn't change the existing Makefile, which is exactly the same whether running locally or in a container
• Added file is also very simple
• Doesn't require extra control variables (LRN_SDK_NO_DOCKER)
• Allows making targets that don't require Python (notably clean) without a container
• Safe with Docker in Docker

Cons

• Easy to forget to add -f docker.mk—the choice has to be expressed every time make is invoked
• Opt-in rather than opt-out
• Extra file

Checklist

• Feature

• Bug

• Security

• Documentation

• ChangeLog.md updated

• Tests added

• All testsuites passed

• make dist completed successfully

[shtrom]

My preference is for Option 1 in #57. The changes are essentially similar, so I'll put my comments there.

[david-scarratt-lrn]

After discussion, the preference overall is for the other option, so closing this.