Optimize context sent for docker build (apache#17415)

The `provider_packages` folder is not needed during docker build
and it often contains copied sources when building the provider
packages. Also _doctree folder is prepared during documentation
building. This makes `docker build` wait for a few seconds when
those directories have a lot of files.

Excluding those decreases docker build overhead significantly.

.dockerignore

@@ -40,9 +40,6 @@
 !scripts/in_container
 !scripts/docker
 
-# Add provider packages to the context
-!provider_packages
-
 # Add tests and kubernetes_tests to context.
 !tests
 !kubernetes_tests
@@ -129,3 +126,4 @@ airflow/www/static/docs
 # Exclude docs generated files
 docs/_build/
 docs/_api/
+docs/_doctrees/