docker/lxc: replace CHATMAIL_NO* env vars with systemd-detect-virt detection

Cherry-pick runtime container detection from the hpk/lxcdeploy branch
for the dovecot sysctl block and apply the same pattern to the
port-availability check in deployers.py, and drop both CHATMAIL_NOSYSCTL
and CHATMAIL_NOPORTCHECK from the container startup script and
Dockerfile; the code now self-detects the context.

docker/chatmail-init.sh

@@ -2,8 +2,6 @@
 
 set -euo pipefail
 export CHATMAIL_INI="${CHATMAIL_INI:-/etc/chatmail/chatmail.ini}"
-export CHATMAIL_NOSYSCTL=True
-export CHATMAIL_NOPORTCHECK=True
 
 CMDEPLOY=/opt/cmdeploy/bin/cmdeploy
 

docker/chatmail_relay.dockerfile

@@ -52,8 +52,6 @@ RUN printf '[params]\nmail_domain = build.local\n' > /tmp/chatmail.ini
 
 RUN CMDEPLOY_STAGES=install \
     CHATMAIL_INI=/tmp/chatmail.ini \
-    CHATMAIL_NOSYSCTL=True \
-    CHATMAIL_NOPORTCHECK=True \
     /opt/cmdeploy/bin/pyinfra @local \
         /opt/chatmail/cmdeploy/src/cmdeploy/run.py -y