As @mscdex has pointed out, libicu was looking for the libicu52 package. Somehow the repository got updated allowing me to pull the new libicu which depends on libicu52 that isn't available in the repository of 12.04, but in 14.04. Since there is no official trusted build of 14.04 in the docker registry, I made my own "base" ubuntu14.04 docker image which starts with 13.10 and upgrades to 14.04;
FROM ubuntu:saucy
ENV DEBIAN_FRONTEND noninteractive
# Work around initramfs-tools running on kernel 'upgrade': <http://bugs.debian.org/cgi- bin/bugreport.cgi?bug=594189>
ENV INITRD No
# Update OS.
RUN sed -i 's/saucy/trusty/g' /etc/apt/sources.list
RUN apt-get update -y
RUN apt-get upgrade -y
RUN apt-get dist-upgrade -y
# Install basic packages.
RUN apt-get install -y software-properties-common
RUN apt-get install -y curl git htop unzip vim wget
# Add files.
ADD root/.bashrc /root/.bashrc
ADD root/.gitconfig /root/.gitconfig
ADD root/scripts /root/scripts
RUN apt-get clean
# Set working directory.
ENV HOME /root
WORKDIR /root
CMD ["/bin/bash"]
Then in the Dockerfile of my worker, I installed libicu52 instead of libicu48 thus fixing all issues