Symlink yarn directory

this will allow for an easier upgrade/downgrade of yarn, e.g:

```
FROM node:6-slim

ENV YARN_VERSION 1.5.1

RUN curl -fSLO --compressed
"https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz"
&&\
    tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/ &&\
    ln -snf /opt/yarn-v$YARN_VERSION /opt/yarn &&\
    rm yarn-v$YARN_VERSION.tar.gz
```

Author: Eyal Zekaria

Dockerfile-alpine.template

@@ -58,8 +58,8 @@ RUN apk add --no-cache --virtual .build-deps-yarn curl gnupg tar \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz" \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz.asc" \
   && gpg --batch --verify yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz \
-  && mkdir -p /opt/yarn \
-  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/yarn --strip-components=1 \
+  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/ \
+  && ln -s /opt/yarn-v$YARN_VERSION /opt/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarnpkg \
   && rm yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz \

Dockerfile-slim.template

@@ -58,8 +58,8 @@ RUN set -ex \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz" \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz.asc" \
   && gpg --batch --verify yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz \
-  && mkdir -p /opt/yarn \
-  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/yarn --strip-components=1 \
+  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/ \
+  && ln -s /opt/yarn-v$YARN_VERSION /opt/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarnpkg \
   && rm yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz

Dockerfile-stretch.template

@@ -53,8 +53,8 @@ RUN set -ex \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz" \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz.asc" \
   && gpg --batch --verify yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz \
-  && mkdir -p /opt/yarn \
-  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/yarn --strip-components=1 \
+  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/ \
+  && ln -s /opt/yarn-v$YARN_VERSION /opt/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarnpkg \
   && rm yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz

Dockerfile-wheezy.template

@@ -49,8 +49,8 @@ RUN set -ex \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz" \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz.asc" \
   && gpg --batch --verify yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz \
-  && mkdir -p /opt/yarn \
-  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/yarn --strip-components=1 \
+  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/ \
+  && ln -s /opt/yarn-v$YARN_VERSION /opt/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarnpkg \
   && rm yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz

Dockerfile.template

@@ -53,8 +53,8 @@ RUN set -ex \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz" \
   && curl -fSLO --compressed "https://yarnpkg.com/downloads/$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz.asc" \
   && gpg --batch --verify yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz \
-  && mkdir -p /opt/yarn \
-  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/yarn --strip-components=1 \
+  && tar -xzf yarn-v$YARN_VERSION.tar.gz -C /opt/ \
+  && ln -s /opt/yarn-v$YARN_VERSION /opt/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn \
   && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarnpkg \
   && rm yarn-v$YARN_VERSION.tar.gz.asc yarn-v$YARN_VERSION.tar.gz