(cherry picked from commit a54a784168c023d1f95c2c1ba0cd97659ffbd08c)

# Conflicts:
#	core/admin/Dockerfile
#	core/dovecot/Dockerfile
#	core/nginx/Dockerfile
#	core/none/Dockerfile
#	core/postfix/Dockerfile
#	optional/clamav/Dockerfile
#	optional/fetchmail/Dockerfile
#	optional/radicale/Dockerfile
#	optional/unbound/Dockerfile
#	setup/Dockerfile

- Moved run to bottom of Dockerfile to allow using unmodified / cached states.
- Simplified bash code in deploy.sh.
- Improved the large bash one-liner in CI.yml. It could not handle >9 for 1.x.

Run fetchmail as root. This is unfortunately required because
all files are owned by root in the mailu data folder.
In the future  we must switch all images to running all
all processes with a non-root user.

alpine3:12 provides fetchmail 6.4.5, so building fetchmail from alpha branch
is not needed anymore (ssl wrapped mode issue fixed) since 6.4.2

This reverts commit c787e4bd.

As per @hoellens suggestion

Co-Authored-By: hoellen <hoellen@users.noreply.github.com>

Fetchmail in alpine is ~5 years old — and doesn’t support current SSL/TLS
variants anymore. This especially leads to our own fetchmail not being able to
pull mail from mailu itself. Since no new fetchmail release is on the horizon,
let’s build the lastest distribution artifact — which strangely is not
6.4.0-snapshot, but 7.0.0-alpha — ourselves.

This fix https://github.com/Mailu/Mailu/issues/918

Bash shell is used by default in Kubernetes' dashboard console, which is very
useful for admins.