Simplify/speed up :demo:buildDocker task

[ericljiang]

The demo subproject's buildDocker task installs gradle on every invocation and the docker image contains an installation of graalvm that is not used.

aws-lambda-graalvm/demo/build.gradle

Lines 40 to 43 in 8102fa1

	task buildDocker(type: Exec) {
	dependsOn classes
	commandLine './buildDocker.sh'
	}

aws-lambda-graalvm/demo/build.Dockerfile

Lines 1 to 9 in 8102fa1

	FROM amazonlinux
	RUN yum install -y which tar gzip gcc glibc-devel zlib-devel wget
	RUN amazon-linux-extras install java-openjdk11
	RUN export JAVA_HOME=$(readlink -f `which java`)
	RUN wget -nv -O /tmp/graalvm.tar.gz https://github.com/oracle/graal/releases/download/vm-19.2.1/graalvm-ce-linux-amd64-19.2.1.tar.gz \
	&& tar -xzf /tmp/graalvm.tar.gz -C /opt \
	&& rm /tmp/graalvm.tar.gz
	ENV GRAALVM_HOME="/opt/graalvm-ce-19.2.1"
	RUN $GRAALVM_HOME/bin/gu install native-image

It can use a gradle docker image to avoid downloading gradle every time. The installation of graalvm can be removed from the docker image or the task can be rewritten to use the preinstalled graalvm instead of downloading it.