Have .dockerignore support Dockerfile/.dockerignore

If .dockerignore mentions either then the client will send them to the
daemon but the daemon will erase them after the Dockerfile has been parsed
to simulate them never being sent in the first place.

an events test kept failing for me so I tried to fix that too

Closes #8330

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/sources/reference/builder.md

@@ -154,6 +154,12 @@ Exclusion patterns match files or directories relative to the source repository
 that will be excluded from the context. Globbing is done using Go's
 [filepath.Match](http://golang.org/pkg/path/filepath#Match) rules.
 
+> **Note**:
+> The `.dockerignore` file can even be used to ignore the `Dockerfile` and
+> `.dockerignore` files. This might be useful if you are copying files from
+> the root of the build context into your new containter but do not want to 
+> include the `Dockerfile` or `.dockerignore` files (e.g. `ADD . /someDir/`).
+
 The following 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.