Error when rebuilding docker container

I think I might have found a fix for #123.

#### Add IMP user
RUN groupadd imp && useradd -g imp -d /home/imp imp \
    && chown imp:imp -R /home/imp/ \
    && chmod -R 0777 /home/imp \
    && echo 'imp:imp' |chpasswd \
    && echo "imp ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers.d/imp \
    && chmod 0440 /etc/sudoers.d/imp # <-- I added this line here

The last line for this code chunk.

However, I could not test it because I could not rebuild the docker container according to the instructions of BUILD_TARBALL.md.

This step fails:

$ docker build -t docker-r3lab.uni.lu/imp/imp:1.4.1.1 .                                                   
Sending build context to Docker daemon  43.01kB
Step 1/18 : FROM docker-r3lab.uni.lu/imp/imp-tools:1.4.1.1
Get https://docker-r3lab.uni.lu/v2/imp/imp-tools/manifests/1.4.1.1: no basic auth credentials

How can I get access to that?