fix(dockerfile): run npm link

Fixes artilleryio#1394.

The currently built container image works fine if you want to
invoke artillery through a single docker command. But if you want to run
multiple artillery commands in the same container instance, or if you
want to use Artillery's container image as a base layer, then you ran
into issues such artilleryio#1394.

The issue us that the executable in the path is named `run` instead of
`artillery`. To address this while maintain compatibility for any users
depending on the existing executable we can run an `npm install` which
will setup artillery as if it had been installed globally.

Dockerfile

@@ -7,6 +7,9 @@ COPY package*.json ./
 RUN npm --ignore-scripts --production install
 
 COPY . ./
+RUN npm link
+
+# keep for backwards compatibility
 ENV PATH="/home/node/artillery/bin:${PATH}"
 
-ENTRYPOINT ["/home/node/artillery/bin/run"]
+ENTRYPOINT ["/usr/local/bin/artillery"]