Don't install yarn manually, it's included in the official images

Closes #3

Dockerfile.onbuild

@@ -7,10 +7,9 @@ ONBUILD COPY package.json yarn.lock* .npmrc* npm-shrinkwrap.json* ./
 # This is in one giant command to keep the image size small
 # TODO: When Finnbuild uses Docker 1.13, we can use --squash, which means this won't have to be one giant command
 ONBUILD RUN apk add --no-cache --virtual build-dependencies make gcc g++ python git && \
-    npm install --loglevel warn --global yarn && \
     # This script does `yarn install` if a `yarn.lock` file is present, otherwise `npm install`
     install-dependencies.sh && \
-    npm uninstall --loglevel warn --global yarn && npm uninstall --loglevel warn --global npm && \
+    rm /usr/local/bin/yarn && npm uninstall --loglevel warn --global npm && \
     apk del build-dependencies
 
 ONBUILD COPY . .

Dockerfile.test

@@ -6,6 +6,4 @@ ENV NODE_ENV test
 # Install dependencies for native builds and yarn
 RUN apk add --no-cache make gcc g++ python git
 
-RUN npm install --global --loglevel warn yarn
-
 CMD ["npm", "test"]

README.md

@@ -18,10 +18,9 @@ COPY package.json yarn.lock* .npmrc* npm-shrinkwrap.json* ./
 # This is in one giant command to keep the image size small
 # NOTE: `install-dependencies.sh` only installs production dependencies, make sure you do transpiling/bundling outside of the image
 RUN apk add --no-cache --virtual build-dependencies make gcc g++ python git && \
-    npm install --global yarn && \
     # This script does `yarn install` if a `yarn.lock` file is present, otherwise `npm install`
     install-dependencies.sh && \
-    npm uninstall --global yarn && npm uninstall --global npm && \
+    rm /usr/local/bin/yarn && npm uninstall --global npm && \
     apk del build-dependencies
 
 COPY . .
@@ -73,7 +72,8 @@ Changes might include:
 
 ## Testing
 
-The normal docker image shouldn't be used for tests, use `containers.schibsted.io/finntech/node:test-<version>` or `containers.schibsted.io/finntech/node:test-onbuild-<version>`.
+The normal docker image shouldn't be used for tests, use `containers.schibsted.io/finntech/node:test-<version>` or
+`containers.schibsted.io/finntech/node:test-onbuild-<version>`.
 
 Dockerfile.test:
 ```Dockerfile