Make building from a git archive the default

If no context is specified and we are in a git repo, then we'll build
from a git archive by default. This means we don't need a separate
setting and gives us a safer default build.

lib/kamal/configuration/builder.rb

@@ -86,7 +86,7 @@ class Kamal::Configuration::Builder
   end
 
   def git_archive?
-    @options["git_archive"]
+    Kamal::Git.used? && @options["context"].nil?
   end
 
   private
@@ -94,9 +94,6 @@ class Kamal::Configuration::Builder
       if @options["cache"] && @options["cache"]["type"]
         raise ArgumentError, "Invalid cache type: #{@options["cache"]["type"]}" unless [ "gha", "registry" ].include?(@options["cache"]["type"])
       end
-      if @options["context"] && @options["git_archive"]
-        raise ArgumentError, "Cannot set a builder context when building from a git archive"
-      end
     end
 
     def cache_image