Use debian bullseye as base image for ddev-webimage, downgrades nginx to 1.18.0, fixes #3116

[rfay]

The Problem/Issue/Bug:

Platformsh/platform SEGV, Acquia ACLI SEGV, curl to certain hosts. Affects only Mac M1 users

• Mac M1 processor on Docker: Segmentation fault on db:dump and user:list platformsh/legacy-cli#1063
• Acquia acli auth:login command and Platform.sh platform db:dump Segmentation fault on Mac M1 #3116

On macOS M1 architectures, the use of curl on some websites causes a SEGV in the web container, see docker desktop 3.4.0 release notes

On Apple Silicon in native arm64 containers, older versions of libssl in debian:buster, ubuntu:20.04 and centos:8 will segfault when connected to some TLS servers, for example curl https://dl.yarnpkg.com. The bug is fixed in newer versions of libssl in debian:bullseye, ubuntu:21.04 and fedora:35

How this PR Solves The Problem:

• Upgrades base image to debian:bullseye-slim
• DOWNGRADES nginx to 1.18 from 1.20 because nginx packages for bullseye are not available. (Request ticket for bullseye packages)

Manual Testing Instructions:

People who are having SEGV trouble on Mac M1 can just add

webimage: drud/ddev-webserver:20210714_bullseye_web_image

to their .ddev/config.yaml (or to a .ddev/config.m1.yaml)

And it should solve problems with curl, the acli program from Acquia, and the platform tool from Platform.sh

Automated Testing Overview:

Related Issue Link(s):

Release/Deployment notes:

• I was unable to build the upstream image in ddev-images using CircleCI - had to build and push it manually on arm64 M1 machine. Very strange.
• Because of the downgrade of nginx from 1.20 to 1.18, I don't think this can be released until those packages are available

[gitpod-io]

[rfay]

I think the path forward for this one is to use the bullseye image only for mac M1 right now, we'll have to build two images.

Also looks like it picked up some inappropriate commits from another PR.