Support a dynamic primary_web_role instead of assuming it's 'web'.

This allows for more meaningful naming in roles.

The only caution here is that we don't support the renaming of roles, so
any migration is left to the user.

lib/kamal/commands/healthcheck.rb

@@ -1,7 +1,7 @@
 class Kamal::Commands::Healthcheck < Kamal::Commands::Base
 
   def run
-    web = config.role(:web)
+    web = config.role(config.primary_web_role)
 
     docker :run,
       "--detach",

lib/kamal/configuration.rb

@@ -90,14 +90,21 @@ class Kamal::Configuration
   end
 
   def primary_web_host
-    role(:web).primary_host
+    role(primary_web_role)&.primary_host
+  end
+
+  def traefik_roles
+    roles.select(&:running_traefik?)
+  end
+
+  def traefik_role_names
+    traefik_roles.flat_map(&:name)
   end
 
   def traefik_hosts
-    roles.select(&:running_traefik?).flat_map(&:hosts).uniq
+    traefik_roles.flat_map(&:hosts).uniq
   end
 
-
   def repository
     [ raw_config.registry["server"], image ].compact.join("/")
   end
@@ -199,6 +206,9 @@ class Kamal::Configuration
     raw_config.asset_path
   end
 
+  def primary_web_role
+    raw_config.primary_web_role || "web"
+  end
 
   def valid?
     ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version
@@ -253,6 +263,10 @@ class Kamal::Configuration
         end
       end
 
+      unless traefik_role_names.include?(primary_web_role)
+        raise ArgumentError, "Role #{primary_web_role} needs to have traefik enabled"
+      end
+
       true
     end