feat: permit building ecs-init that will run inside a strict snap

[stuart-warren]

Summary

Permits the binary to work within a strictly confined snap

ref: aws/containers-roadmap#1487

Implementation details

• add go build flag snap

• add make target snap-compatible

• add config.ResolveDirectory() and config.ResolveDockerDirectory() to add prefixes to paths where required

• add config.HomeDirectory() due to snaps having a different structure and different home directory

• only mount host directories into agent container that exist due to read only filesystem

Testing

Building a strictly confined snap and running on Ubuntu Core 20

New tests cover the changes: no

Description for the changelog

Feature - enable functionality within a strictly confined snap when built with snap build flag

Licensing

This contribution is under the terms of the Apache 2.0 License: yes

[stuart-warren]

currently failing tests due to my change to only mount existing directories.

discussing changes in separate issue aws/amazon-ecs-agent#3152

https://github.com/aws/amazon-ecs-init/blob/d5b2b34772c48306a5ee6d8fb82cd0cc0267e7ad/ecs-init/docker/docker_test.go#L261-L285

https://github.com/aws/amazon-ecs-init/blob/d5b2b34772c48306a5ee6d8fb82cd0cc0267e7ad/ecs-init/docker/docker_test.go#L317-L321