feat: user multi-stage builds and remove apt and pip caches

[LucasRoesler]

Recreated for Hacktoberfest ;)

1. Remove apt cache after running apt-get
2. Pass --no-cache-dir to the pip command
3. Use multi-stage builds to remove the test layers from the final image.

Motivation and Context

• [x] I have raised an issue to propose this change (required)
• [ ] My issue has received approval from the maintainers or lead with the design/approved label

Resolves #57 Relates to #55

How Has This Been Tested?

$ faas-cli new echo0 --lang python3-flask 
$ faas-cli new echo1 --lang python3-http

$ faas-cli build -f echo0.yml --no-cache
# truncated
Image: echo0:latest built.
[0] < Building echo0 done in 37.65s.
[0] Worker done.

Total build time: 37.65s

$ faas-cli build -f echo1.yml --no-cache
# truncated
Image: echo1:latest built.
[0] < Building echo1 done in 32.50s.
[0] Worker done.

Total build time: 32.50s

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[alexellis]

I'll get this merged, thank you so much. Could you send similar to the Python templates in openfaas/templates?