Fix docker image build - add python3

### Motivation
Building the docker image does not work anymore, see #2601 

### Changes
Add python3 to the list of available packages

## How to verify
cd docker
docker build --build-arg BK_VERSION=4.12.1 -t testbk .

Fixes #2601 

Reviewers: Andrey Yegorov <None>

This closes #2609 from eolivelli/fix/docker-image-python3
diff --git a/docker/Dockerfile b/docker/Dockerfile
index 5676852..a9baa5e 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -33,7 +33,7 @@
 # Download Apache Bookkeeper, untar and clean up
 RUN set -x \
     && adduser "${BK_USER}" \
-    && yum install -y java-11-openjdk-devel wget bash python sudo\
+    && yum install -y java-11-openjdk-devel wget bash python3 sudo\
     && mkdir -pv /opt \
     && cd /opt \
     && wget -q "${DISTRO_URL}" \
@@ -48,6 +48,7 @@
     && rm -rf "$DISTRO_NAME.tar.gz" "$DISTRO_NAME.tar.gz.asc" "$DISTRO_NAME.tar.gz.sha512" \
     # install zookeeper shell
     && wget -q https://bootstrap.pypa.io/2.7/get-pip.py \
+    && python --version \
     && python get-pip.py \
     && pip install zk-shell \
     && rm -rf get-pip.py \