Fix pip command name

Author: tatsushid

3.4/Dockerfile

@@ -93,7 +93,7 @@ RUN tce-load -wic \
     && sudo /sbin/ldconfig \
     && rm -rf /tmp/tce/optional/* \
     && curl -SL 'https://bootstrap.pypa.io/get-pip.py' | sudo python3 \
-    && sudo pip install --no-cache-dir --upgrade pip==$PYTHON_PIP_VERSION \
+    && sudo pip3 install --no-cache-dir --upgrade pip==$PYTHON_PIP_VERSION \
     && sudo find /usr/local \( -type f -a -name '*.pyc' -o -name '*.pyo' \) -exec rm -rf '{}' + \
     && find /usr/local \( -type d -a -name test -o -name tests \) | sudo xargs rm -rf \
     && sudo rm -rf /usr/src/python