replace command syntax with usage of file provider to set skip verify of self signed certs

.env.sample

@@ -150,3 +150,4 @@ COMPOSE_FILE="compose.yml"
 
 ## Zulip
 #COMPOSE_FILE="$COMPOSE_FILE:compose.zulip.yml"
+#ZULIP_HTTPS_PROXY_ENABLED=1

compose.zulip.yml

@@ -1,5 +1,6 @@
 version: "3.8"
 services:
   app:
-    command:
-      - "--serversTransport.insecureSkipVerify=true"
+    deploy:
+      labels:
+        - "traefik.http.services.${STACK_NAME}.loadBalancer.serversTransport=zulip@file"

file-provider.yml.tmpl

@@ -1,5 +1,10 @@
 ---
 http:
+    {{ if eq (env "ZULIP_HTTPS_PROXY_ENABLED") "1" }}
+  serversTransports:
+    zulip:
+      insecureSkipVerify: true
+  {{ end }}
   middlewares:
     {{ if eq (env "KEYCLOAK_MIDDLEWARE_ENABLED") "1" }}
     keycloak: