I managed to get it working! I had to add ** USER root ** in my dockerfile as it was running under the wrong user: ** USER 1001 **, which came from rasa’s dockerfile.
I share the full dockerfile in case someone runs into the same problem in the future:
FROM rasa/rasa:1.10.12-spacy-en
USER root
RUN python -m spacy download es_core_news_md && \
python -m spacy link es_core_news_md es
Thank you @jusce17