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

[LucasRoesler]

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

• 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

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

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• 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]

Hi @LucasRoesler could you rebase this PR? You'll be at 2/4 for Hacktoberfest eh? 😄

[LucasRoesler]

@alexellis done

fyi, i think the PR creation date has to be in October.. so still 0/4 ;)

[alexellis]

Close it and submit another one if you like?