Add a simple validation to the service name to prevent setup issues

lib/kamal/configuration.rb

@@ -218,7 +218,7 @@ class Kamal::Configuration
 
 
   def valid?
-    ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version
+    ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version && ensure_valid_service_name
   end
 
   def to_h
@@ -283,6 +283,12 @@ class Kamal::Configuration
       true
     end
 
+    def ensure_valid_service_name
+      raise ArgumentError, "Service name can only include alphanumeric characters and hyphens" unless raw_config[:service] =~ /^[a-z0-9-]+$/
+
+      true
+    end
+
     def ensure_valid_kamal_version
       if minimum_version && Gem::Version.new(minimum_version) > Gem::Version.new(Kamal::VERSION)
         raise ArgumentError, "Current version is #{Kamal::VERSION}, minimum required is #{minimum_version}"