Use docker-compose build for dynamic containers, allow easier container config

[rfay]

The Problem/Issue/Bug:

• Our web container gets bigger and bigger
• Users will always have new requests for containers, or for extra config

How this PR Solves The Problem:

• Uses docker-compose up --build, so that a build context can be added for dynamic containers
• If .ddev/db-build/Dockerfile or .ddev/web-build/Dockerfile exists, the Dockerfile there will be used to build the web or db image.
• config.yaml webimage_extra_packages and dbimage_extra_packages allow just adding debian packages without writing a Dockerfile.
• See the docs

TODO:

• Allow a simple list of packages to add in config.yaml
• docs
• .ddev/*-build/Dockerfile.example should be gitignored
• Add config commands for *_extra_packages

Manual Testing Instructions:

• Create a .ddev/web-build/Dockerfile with these contents:

ARG BASE_IMAGE=drud/ddev-webserver:latest
FROM $BASE_IMAGE
RUN touch /var/tmp/this-was-from
RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y php7.3-yaml

• ddev start

• You should find on ddev ssh that php7.3-yaml is installed

• Do the same thing for db-build (with a different package or action)

Automated Testing Overview:

• Test webimage_extra_packages and dbimage_extra_packages
• Test to make sure Dockerfile overrides webimage_extra_packages
• Test behavior of Dockerfile

Related Issue Link(s):

Release/Deployment notes:

[andrewfrench]

This is very interesting, and I expect this will be used as a more robust replacement for complex pre-start hook configurations? I think it's a great and pretty powerful idea and I'm interested in seeing how people end up using this.

While testing, I confirmed that I could define a Dockerfile used to extend both the ddev-webserver and ddev-dbserver containers, build them on start, and confirm that the customizations made in the custom Dockerfile were present in the container using ddev ssh.