[docs][docker] cleaned up CODEOWNERS and fixed build warning in dockerfile-python (#2555)

.github/CODEOWNERS

@@ -6,6 +6,9 @@
 # setting reviewers on PRs manually, but this file should
 # offer a reasonable automatic best-guess
 
+# catch-all rule (this only gets matched if no rules below match)
+*    @guolinke @StrikerRUS @jameslamb @Laurae2
+
 # main C++ code
 include/    @guolinke @chivee
 src/    @guolinke @chivee
@@ -27,11 +30,18 @@ helpers/    @StrikerRUS @guolinke
 # CI administrative stuff
 .ci/    @StrikerRUS @Laurae2 @jameslamb
 docs/    @StrikerRUS @Laurae2 @jameslamb
+examples/     @StrikerRUS @jameslamb @guolinke
 *.yml    @StrikerRUS @Laurae2 @jameslamb
 .vsts-ci.yml    @Laurae2
 
-# GPU code
+# docker setup
+docker/    @StrikerRUS @jameslamb
+docker/dockerfile-cli    @guolinke @chivee
 docker/gpu/    @huanzhang12
+docker/dockerfile-python    @StrikerRUS @chivee @wxchan @henry0312
+docker/dockerfile-r    @Laurae2 @jameslamb
+
+# GPU code
 docs/GPU-*.rst    @huanzhang12
 src/treelearner/gpu_tree_learner.cpp    @huanzhang12 @guolinke @chivee
 src/treelearner/tree_learner.cpp    @huanzhang12 @guolinke @chivee

docker/dockerfile-python

@@ -12,9 +12,7 @@ RUN apt-get update && \
         g++ \
         git \
         wget && \
-
-# python-package
-    # miniconda
+    # python environment
     wget https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh && \
     /bin/bash Miniconda3-latest-Linux-x86_64.sh -f -b -p $CONDA_DIR && \
     export PATH="$CONDA_DIR/bin:$PATH" && \
@@ -23,8 +21,7 @@ RUN apt-get update && \
     conda install -q -y numpy scipy scikit-learn pandas && \
     git clone --recursive --branch stable --depth 1 https://github.com/Microsoft/LightGBM && \
     cd LightGBM/python-package && python setup.py install && \
-
-# clean
+    # clean
     apt-get autoremove -y && apt-get clean && \
     conda clean -a -y && \
     rm -rf /usr/local/src/*

docker/dockerfile-r

@@ -2,7 +2,6 @@ FROM rocker/verse:latest
 
 WORKDIR /lgbm
 
-# R
 RUN apt-get update && \
     apt-get install -y build-essential && \
     git clone --recursive --branch stable --depth 1 https://github.com/Microsoft/LightGBM && \