Update versions incude fix for mike's set-default

- CentOS 7.6.1810
- mkdocs 1.0.4
- mike 0.3.5 (includes fix for set-default)

Author: PhilippSalvisberg

docker/Dockerfile

@@ -1,23 +1,28 @@
-FROM centos:7.5.1804
+FROM centos:7.6.1810
 
 LABEL maintainer="philipp.salvisberg@trivadis.com"
 LABEL description="Tools to generate HTML and PDF using Materials for MkDocs and wkhtmltopdf."
 LABEL build.command="docker build . --tag trivadis/mktools:latest"
 
+# install python, not part of centos
+# see https://www.osetc.com/en/how-to-install-the-latest-python-3-3-6-x-3-7-x-on-centos-7-or-rhel-7.html
 RUN yum install -y https://centos7.iuscommunity.org/ius-release.rpm
 RUN yum update -y
 RUN yum install -y python36u python36u-libs python36u-devel python36u-pip
+
+# install git and wget
 RUN yum install -y git
 RUN yum install -y wget
 
-# Install wkhtmltox 0.12.4 since 0.12.5 cannot create TOCs, see https://github.com/wkhtmltopdf/wkhtmltopdf/issues/3995
+# install wkhtmltox 0.12.4 since 0.12.5 cannot create TOCs, see https://github.com/wkhtmltopdf/wkhtmltopdf/issues/3995
 # RUN yum install -y https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.5/wkhtmltox-0.12.5-1.centos7.x86_64.rpm
 RUN yum install -y yum install -y wkhtmltopdf
 RUN wget -q https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.4/wkhtmltox-0.12.4_linux-generic-amd64.tar.xz -O /tmp/wkhtmltox.tar.xz && \
     tar xvf /tmp/wkhtmltox.tar.xz -C /tmp && \
     cp -rp /tmp/wkhtmltox/* /usr/local && \
     rm -rf /tmp/wkhtmltox*
 
+# install python modules (most recent versions)
 RUN pip3.6 install --upgrade pip
 RUN pip3.6 install mkdocs \
                    mkdocs-material \