Note .dockerignore in the exclusion for credentials (#376)

Closes #375

(Opening as me to trigger CI)

---------

Signed-off-by: Andrew Howden <hello@andrewhowden.com>
Signed-off-by: Seth Vargo <seth@sethvargo.com>
Co-authored-by: Andrew Howden <hello@andrewhowden.com>

README.md

@@ -27,8 +27,8 @@ support](https://cloud.google.com/support).**
     unable to authenticate.
 
 -   To create binaries, containers, pull requests, or other releases, add the
-    following to your `.gitignore` to prevent accidentially committing
-    credentials to your release artifact:
+    following to your `.gitignore`, `.dockerignore` and similar files to prevent
+    accidentally committing credentials to your release artifact:
 
     ```text
     # Ignore generated credentials from google-github-actions/auth