add dockerfile and documentation

README.txt

@@ -28,6 +28,19 @@ Source Directories
 xar -
         Primary source directory, contains libxar sources and xar sources.
 
+docker -
+        Run xar encapsulated in docker to do use it in your file system:
+
+        ```
+        cd docker
+        sudo docker build -t xar .
+        ```
+
+        then for example if you wish to extract a pkg file
+        ```
+        sudo docker run -ti --rm -v path_to_pkg_file:/xar/ xar xar -xvf packagefile.pkg
+        ```
+
 XarCMPlugin -
         OS X xar contextual menu plugin with Xcode project.  May be out of date.
 

docker/Dockerfile

@@ -0,0 +1,15 @@
+FROM ubuntu:20.10
+
+RUN apt-get update && apt-get install -y git build-essential autoconf libxml2-dev vim libssl-dev libz-dev
+RUN useradd -ms /bin/bash xar
+RUN git clone https://github.com/mackyle/xar.git 
+RUN cd /xar/xar/; \
+    # Based on https://askubuntu.com/questions/1108506/building-xar-source-error-no-libcrypto :
+    sed -i '332s/^.*$/AC_CHECK_LIB([crypto], [OPENSSL_init_crypto], , [have_libcrypto="0"])/' configure.ac; \
+    ./autogen.sh; \
+    ./configure; \
+    make; \
+    make install
+RUN chown -R xar:xar /xar/
+USER xar
+WORKDIR /xar/