Refactor Docker setup for smaller/ faster builds

[grahamalama]

• use multi-stage build process. Only download build dependencies in intermediate stage
• add .dockerignore for dist/test images
• reorder build steps for better layer caching

Also adds build script to build and tag dist, test images.

Size comparison of builds. Images with -before were built with code present in main (prior to this PR):

kinto               build          71db14f4f99a   5 minutes ago    349MB
kinto               tests          0debcb48417c   35 minutes ago   371MB
kinto               tests-before   2f90fb699378   35 minutes ago   1.4GB
kinto               build-before   8de540b3e6b2   37 minutes ago   1.37GB

Also, compare build times between the test suite run on the main branch before this PR and after.

Before:

build_test_publish   4m 37s
integration test     3m 2s

After:

build_test_publish   1m 59s
integration test     1m 49s

[grahamalama]

Thanks for the approvals! I'm going to squash these commits then merge with bors

[grahamalama]

bors r+

[bors[bot]]

Build succeeded:

• ci/circleci: build_test_publish