update .dockerignore to prevent '-unsupported' builds

When building the daemon inside the development container,
without bind-mounting the source-code, the git status was
marked "dirty", because the `.dockerignore` and `Dockerfile`
were not copied into the image (due to them being excluded
by the .dockerignore):

```
make shell
hack/make.sh binary
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 GITCOMMIT = 98276a3439-unsupported
 The version you are building is listed as unsupported because
 there are some files in the git repository that are in an uncommitted state.
 Commit these changes, or add to .gitignore to remove the -unsupported from the version.
 Here is the current list:
 D .dockerignore
 D Dockerfile
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

This patch removes those files from .dockerignore so that the
git status is not marked "dirty".

Excluding the files through `.dockerignore` is only useful to
prevent busting the build-cache in corner-cases where the files
are modified without actual changes (e.g. whitespace-only changes,
comment edited), but should not be a big issue in practice.

With this patch applied:

```
make shell
hack/make.sh binary

Removing bundles/

---> Making bundle: binary (in bundles/binary)
Building: bundles/binary-daemon/dockerd-dev
GOOS="" GOARCH="" GOARM=""
Created binary: bundles/binary-daemon/dockerd-dev
Copying nested executables into bundles/binary-daemon

bundles/binary-daemon/dockerd-dev --version
Docker version dev, build 7812dd38c8
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

1 files changed, 0 insertions, 2 deletions

diff --git a/.dockerignore b/.dockerignore
index b8f478b4c1..a8eeb4b41f 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1,8 +1,6 @@
-.dockerignore
 .git
 .go-pkg-cache
 .gopath
 bundles
-Dockerfile
 vendor/pkg