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/cli/templates/deploy.yml

@@ -31,7 +31,6 @@ registry:
 #   user: app
 
 # Configure builder setup.
-# Set git_archive: true to build from a git archive of HEAD
 # builder:
 #   args:
 #     RUBY_VERSION: 3.2.0
@@ -40,7 +39,6 @@ registry:
 #   remote:
 #     arch: amd64
 #     host: ssh://app@192.168.0.1
-#   git_archive: false
 
 # Use accessory services (secrets come from .env).
 # accessories:

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