Add proper makefile and build info

Dockerfile

 FROM openjdk:8-jdk
 
+# define arguments for versions to be used for platform and build tools
+ARG PLATFORM
+ARG BUILD_TOOLS
+
 RUN apt-get --quiet update --yes && \
     apt-get --quiet install --yes wget tar unzip lib32stdc++6 lib32z1
 RUN wget --quiet --output-document=android-sdk.zip https://dl.google.com/android/repository/sdk-tools-linux-4333796.zip && \
@@ -10,7 +14,7 @@ RUN mkdir android-sdk-linux/licenses && \
     printf "84831b9409646a918e30573bab4c9c91346d8abd" > android-sdk-linux/licenses/android-sdk-preview-license
 RUN android-sdk-linux/tools/bin/sdkmanager --update
 RUN android-sdk-linux/tools/bin/sdkmanager \
-        "platforms;android-28" \
-        "build-tools;29.0.1" \
+        "platforms;android-${PLATFORM}" \
+        "build-tools;${BUILD_TOOLS}" \
         "extras;google;m2repository" \
         "extras;android;m2repository"

Makefile

+NAME=k8r.eu/justjanne/android-sdk
+PLATFORM=28
+BUILD_TOOLS=28.0.3
+
+.PHONY: all
+all: push
+
+.PHONY: build
+build: Dockerfile
+	docker build -t $(NAME):$(PLATFORM)-$(BUILD_TOOLS) --build-arg PLATFORM=$(PLATFORM) --build-arg BUILD_TOOLS=$(BUILD_TOOLS) .
+	docker tag $(NAME):$(PLATFORM)-$(BUILD_TOOLS) $(NAME):latest
+
+.PHONY: push
+push: build
+	docker push $(NAME):$(PLATFORM)-$(BUILD_TOOLS)
+	docker push $(NAME):latest
+