autoland-ui: Ignore node_modules in .dockerignore to let yarn image build normally (bug 1344004). r?davidwalsh Commands like docker-compose build or docker-compose up --build (which happen a lot when you need to play with the package.json or yarn) takes forever because docker tries to zip the the developers local node_modules folder (if it exists) folder even though it is never used in building the image. This change speeds things up incredibly. MozReview-Commit-ID: H1IJVqLmu6T

new file mode 100644
--- /dev/null
+++ b/autoland/ui/.dockerignore
@@ -0,0 +1,1 @@
+node_modules
\ No newline at end of file