docs: document dockerignore update

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 4f3bc15817b3f86f8c3d25df2389da253655c071)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 7bfe3d20063f3986fb5c61be48b87c6b8d2b87e9
Component: cli

Author: tonistiigi

components/cli/docs/reference/commandline/build.md

@@ -324,7 +324,16 @@ Successfully built 99cc1ad10469
 This example shows the use of the `.dockerignore` file to exclude the `.git`
 directory from the context. Its effect can be seen in the changed size of the
 uploaded context. The builder reference contains detailed information on
-[creating a .dockerignore file](../builder.md#dockerignore-file)
+[creating a .dockerignore file](../builder.md#dockerignore-file).
+
+When using the [BuildKit backend](../builder.md#buildkit), `docker build` searches
+for a `.dockerignore` file relative to the Dockerfile name. For example, running
+`docker build -f myapp.Dockerfile .` will first look for an ignore file named
+`myapp.Dockerfile.dockerignore`. If such a file is not found, the `.dockerignore`
+file is used if present. Using a Dockerfile based `.dockerignore` is useful if a
+project contains multiple Dockerfiles that expect to ignore different sets of
+files.
+
 
 ### Tag an image (-t)