Fix yarn failure to install (package.json lookup)

yarn failed to install and thus failed to install gateway dependencies,
to we relocate it to let it find gateway's package.json file.

Observed (silent) issue on docker 17.12.1-ce (Ubuntu-18.04) on x86_64:

    npm WARN saveError ENOENT: no such file or directory, open '/home/node/package.json'
    (...)
    + yarn@1.9.4
    added 1 package in 0.447s
    Cloning into 'intent-parser'...
    Cloning into 'gateway'...
    yarn install v1.9.4
    [1/4] Resolving packages...
    [1/4] Resolving packages...
    [2/4] Fetching packages...
    info fsevents@1.2.4: The platform "linux" is incompatible with this module.
    (...)
    [3/4] Linking dependencies...
    [4/4] Building fresh packages...

Change-Id: I8737dfc28da8c8bf266de829725f3c09f62d9468
Signed-off-by: Philippe Coval <p.coval@samsung.com>

Dockerfile

@@ -33,14 +33,15 @@ RUN echo "deb http://ftp.debian.org/debian stretch-backports main" >> /etc/apt/s
     echo '%sudo ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
 
 USER node
-RUN cd /home/node && \
-    npm install yarn && \
+WORKDIR /home/node
+RUN set -x && \
     mkdir mozilla-iot && \
     cd mozilla-iot && \
     git clone https://github.com/mozilla-iot/intent-parser && \
     git clone https://github.com/mozilla-iot/gateway && \
     cd gateway && \
-    /home/node/node_modules/.bin/yarn
+    npm install yarn && \
+    yarn
 
 USER root
 ADD service /etc/service