Fix installing a locally cloned requirement repository

When running `pip install -r /path/to/x.txt`, relative paths listed in
x.txt are supposed to be relative to the current folder, and not the
x.txt folder. This results in an error during `pip install`:

   ./xblock-poll/ should either be a path to a local project or a VCS
   url beginning with svn+, git+, hg+, or bzr+

This error was first reported here:
https://discuss.overhang.io/t/installing-custom-xblock/61

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Latest
 
+- [Bugfix] Fix installing a locally cloned requirement repository
 - [Improvement] Add `--no-cache` option to `images build`
 - [Improvement] Make it possible to configure the notes service hostname
 

tutor/templates/build/openedx/Dockerfile

@@ -58,7 +58,7 @@ ENV PATH ./node_modules/.bin:${PATH}
 
 # Install private requirements: this is useful for installing custom xblocks.
 COPY ./requirements/ /openedx/requirements
-RUN pip install -r /openedx/requirements/private.txt
+RUN cd /openedx/requirements/ && pip install -r ./private.txt
 
 # Create folder that will store *.env.json and *.auth.json files, as well as
 # the tutor-specific settings files.