fix docker script (#93)

* fix test collator compilation failure

Closes #88.

* copy method instead of using feature

Using a feature gate like "test-features" is problematic because
it is leaky: depending on situational considitons such as the
current working directory when compilation is attempted, the
feature may or may not be applied, which makes success inconsistent.
It's simpler in this case to copy a dozen lines of code than to
work out all the issues with test features.

* docker script shoudld work on cumulus-branch (default)

* Add storage_append to runtime

Co-authored-by: Cecile Tonglet <[email protected]>