Merge branch 'fix/prepare-build-in-docker' into 'master'

Fix prepare build execution in docker environment

In newest versions of docker the `/.dockerinit` file doesn't exists any more (https://github.com/docker/docker/pull/19490). And oo our builds are failing, because the `./scripts/prepare_build.sh` script isn't installing required dependencies.

This MR fixes the problem with backward compatibility for oldest docker environments. 

/cc @grzegorz @douwe 

See merge request !1950

scripts/prepare_build.sh

@@ -11,7 +11,7 @@ retry() {
     return 1
 }
 
-if [ -f /.dockerinit ]; then
+if [ -f /.dockerenv ] || [ -f ./dockerinit ]; then
     mkdir -p vendor
 
     # Install phantomjs package