Provide a Docker image for reproducible builds

Motivation:

It would be good to provide a docker image for people that want to build netty on linux.

Modifications:

Add a docker file

Result:

People can more easily build netty. Fixes [#7585].

docker/Dockerfile-netty-centos6

@@ -0,0 +1,34 @@
+FROM centos:6
+MAINTAINER netty@googlegroups.com
+ENTRYPOINT /bin/bash
+
+ENV SOURCE_DIR $HOME/source
+ENV MAVEN_VERSION 3.5.2
+ENV JAVA_VERSION 1.8.0
+
+RUN mkdir $SOURCE_DIR
+WORKDIR $SOURCE_DIR
+
+# install dependencies
+RUN yum install -y \
+ apr-devel \
+ autoconf \
+ automake \
+ git \
+ glibc-devel \
+ java-$JAVA_VERSION-openjdk-devel \
+ libtool \ 
+ lksctp-tools \
+ lsb-core \
+ make \
+ openssl-devel \
+ tar \
+ wget
+
+
+RUN wget -q http://archive.apache.org/dist/maven/maven-3/$MAVEN_VERSION/binaries/apache-maven-$MAVEN_VERSION-bin.tar.gz && tar xfz apache-maven-$MAVEN_VERSION-bin.tar.gz && mv apache-maven-$MAVEN_VERSION /opt/
+
+RUN echo 'PATH=/opt/apache-maven-$MAVEN_VERSION/bin:$PATH' >> ~/.bashrc
+RUN echo 'export JAVA_HOME="/usr/lib/jvm/java-$JAVA_VERSION/"' >> ~/.bashrc
+
+RUN rm -rf $SOURCE_DIR

docker/README.md

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+
+** Create a docker image **
+```
+docker build -f Dockerfile-netty-centos6 . -t netty-centos6
+```
+
+** Using the image **
+
+```
+cd /path/to/netty/
+```
+
+```
+docker run -it -v ~/.m2:/root/.m2 -v ~/.ssh:/root/.ssh -v ~/.gnupg:/root/.gnupg -v `pwd`:/code -w /code netty-centos6 bash
+```