add Dockerfile - custom image for CI, add java8

Dockerfile

@@ -0,0 +1,7 @@
+FROM jangrewe/gitlab-ci-android
+
+RUN apt-get -qq update \
+    && apt-get remove openjdk-11* -qqy \
+    && apt-get install -qqy --no-install-recommends \
+        openjdk-8-jdk \
+    && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*