Fix Docker build when sourced from artifacts

This commit fixes an issue when the artifact used to build the Docker
image is sourced from artifacts.elastic.co. In particular, the artifact
was not downloaded to the proper location.

distribution/docker/build.gradle

@@ -23,7 +23,7 @@ ext.expansions = { oss ->
   return [
     'elasticsearch'       : elasticsearch,
     'license'             : oss ? 'Apache-2.0' : 'Elastic License',
-    'source_elasticsearch': local() ? "COPY $elasticsearch /opt/" : "RUN curl --retry 8 -s -L -O https://artifacts.elastic.co/downloads/elasticsearch/${elasticsearch}",
+    'source_elasticsearch': local() ? "COPY $elasticsearch /opt/" : "RUN cd /opt && curl --retry 8 -s -L -O https://artifacts.elastic.co/downloads/elasticsearch/${elasticsearch} && cd -",
     'version'             : VersionProperties.elasticsearch
   ]
 }