Updates Dockerfile workaround kivy#625 and README.rst, fixes kivy#589

Dockerfile

@@ -25,7 +25,7 @@ ENV LANG="en_US.UTF-8" \
 
 # installs system dependencies (required to setup all the tools)
 RUN apt install -qq --yes --no-install-recommends \
-    sudo python-pip python-setuptools
+    sudo python-pip python-setuptools file
 
 # https://buildozer.readthedocs.io/en/latest/installation.html#android-on-ubuntu-18-04-64bit
 RUN dpkg --add-architecture i386 && apt update -qq > /dev/null && \
@@ -45,4 +45,11 @@ WORKDIR ${WORK_DIR}
 
 # installs buildozer and dependencies
 RUN pip install --user Cython==0.25.2 buildozer
+# calling buildozer adb command should trigger SDK/NDK first install and update
+# but it requires a buildozer.spec file
+RUN cd /tmp/ && buildozer init && buildozer android adb -- version && cd -
+# fixes source and target JDK version, refs https://github.com/kivy/buildozer/issues/625
+RUN sed s/'name="java.source" value="1.5"'/'name="java.source" value="7"'/ -i ${HOME_DIR}/.buildozer/android/platform/android-sdk-20/tools/ant/build.xml
+RUN sed s/'name="java.target" value="1.5"'/'name="java.target" value="7"'/ -i ${HOME_DIR}/.buildozer/android/platform/android-sdk-20/tools/ant/build.xml
+
 ENTRYPOINT ["buildozer"]

README.rst

@@ -76,6 +76,20 @@ The pip package does not yet support python3.
 #.  Please note the "android" buildozer target, and use that for any and all buildozer commands you run (even if the docs just say "android").
 
 
+Buildozer Docker image
+----------------------
+
+A Dockerfile is available to use buildozer through a Docker environment.
+
+#. Build with::
+
+    docker build --tag=buildozer .
+
+#. Run with::
+
+    docker run --volume "$(pwd)":/home/user/hostcwd buildozer --version
+
+
 Examples of Buildozer commands:
 --------------------------------