Added new dockerfile for plugin development

Dockerfile.onbuild

@@ -0,0 +1,13 @@
+FROM coralproject/talk:latest
+
+# Bundle app source
+ONBUILD COPY . /usr/src/app
+
+# At this stage, we need to install the development dependancies again because 
+# we need to have webpack available. We then build the new dependancies and
+# clear out the development dependancies again. After this we of course need to
+# clear out the yarn cache, this saves quite a lot of size.
+ONBUILD RUN NODE_ENV=development yarn install && \
+            NODE_ENV=production  yarn build && \
+            NODE_ENV=production  yarn install --production && \
+            yarn cache clean