refactor: use $DOMAIN env var instead of hardcoded lazyworkhorse.net

Replace all hardcoded lazyworkhorse.net references in compose files
with ${DOMAIN} variable substitution. Create .env.production and
.env.staging environment files. Update Makefile with ENV selection
(--env-file support) and staging/production targets.

Changes:
- All 13 compose YAML files: lazyworkhorse.net -> ${DOMAIN}
- New .env.production (DOMAIN=lazyworkhorse.net)
- New .env.staging (DOMAIN=staging.lazyworkhorse.net)
- Makefile: ENV var, --env-file flag, staging/production targets
- Gitea redirect regex updated for variable substitution
- CI workflow checkout URLs left hardcoded (infrastructure refs)
- Dockerfile SSH host refs left hardcoded (infrastructure refs)

Deploy: make ENV=staging all_up  or  make staging
        make ENV=production all_up  or  make production

authentification/compose.yml

@@ -13,12 +13,12 @@ services:
       - "traefik.enable=true"
 
       # HTTP router
-      - "traefik.http.routers.authelia-http.rule=Host(`auth.lazyworkhorse.net`)"
+      - "traefik.http.routers.authelia-http.rule=Host(`auth.${DOMAIN}`)"
       - "traefik.http.routers.authelia-http.entrypoints=web"
       - "traefik.http.routers.authelia-http.middlewares=redirect-to-https"
 
       # HTTPS router
-      - "traefik.http.routers.authelia-https.rule=Host(`auth.lazyworkhorse.net`)"
+      - "traefik.http.routers.authelia-https.rule=Host(`auth.${DOMAIN}`)"
       - "traefik.http.routers.authelia-https.entrypoints=websecure"
       - "traefik.http.routers.authelia-https.tls=true"
       - "traefik.http.routers.authelia-https.tls.certresolver=njalla"
@@ -26,7 +26,7 @@ services:
       - "traefik.http.services.authelia.loadbalancer.server.port=9091"
 
       # forward auth middleware definition
-      - "traefik.http.middlewares.authelia.forwardauth.address=http://authelia:9091/api/verify?rd=https://auth.lazyworkhorse.net"
+      - "traefik.http.middlewares.authelia.forwardauth.address=http://authelia:9091/api/verify?rd=https://auth.${DOMAIN}"
       - "traefik.http.middlewares.authelia.forwardauth.trustForwardHeader=true"
       - "traefik.http.middlewares.authelia.forwardauth.authResponseHeaders=Remote-User,Remote-Groups,Remote-Name,Remote-Email"