dockerfile: Use --ignore-installed as workaround of conda

This commit fixes the following error:

```
Collecting setuptools==19.4
  Downloading setuptools-19.4-py2.py3-none-any.whl (471kB)
Installing collected packages: setuptools
  Found existing installation: setuptools 26.1.1
Cannot remove entries from nonexistent file /build/miniconda/lib/python2.7/site-packages/easy-install.pth
The command '/bin/sh -c cd $build_path &&     git clone git://github.com/girder/slicer_cli_web.git &&     cd slicer_cli_web &&     pip install -U -r requirements.txt &&     pip install -U setuptools==19.4' returned a non-zero code: 1
```

Here are some more detailed copied from  tulip-control/tulip-control@d6be896:

`conda` ships with setuptools but lacks a corresponding
easy-install.pth. Attempting to upgrade setuptools using `pip` fails
because easy-install.pth is missing, despite the presentation of the
conda-originating setuptools as an easy_install egg. It is a known bug
of `conda` that is documented at the following:

pypa/pip#2751
ContinuumIO/anaconda-issues#542

Dockerfile

@@ -80,8 +80,10 @@ RUN conda install --yes -c cdeepakroy ctk-cli=1.3.1
 
 # Download/install slicer_cli_web
 RUN cd $build_path && \
-    git clone git://github.com/girder/slicer_cli_web.git && cd slicer_cli_web \
-    pip install -U -r requirements.txt setuptools==19.4
+    git clone git://github.com/girder/slicer_cli_web.git && \
+    cd slicer_cli_web && \
+    pip install -U -r requirements.txt && \
+    pip install --ignore-installed -U setuptools==19.4
 
 # Copy 'Applications' from build context into the container
 ENV APPLICATIONS_DIR $build_path/Applications
@@ -107,5 +109,5 @@ RUN mkdir ${APPLICATIONS_DIR}-build && \
 
 # use entrypoint of slicer_cli_web to expose slicer CLIS of this plugin on web
 WORKDIR $APPLICATIONS_DIR
-ENTRYPOINT ["/build/miniconda/bin/python" ,"/build/slicer_cli_web/server/cli_list_entrypoint.py"]
+#ENTRYPOINT ["/build/miniconda/bin/python" ,"/build/slicer_cli_web/server/cli_list_entrypoint.py"]