fix pulsar_standalone docker image build failed (#11862)

### Motivation
When build the pulsar-standalone docker image, it throw the following exception

[INFO] + sudo -u postgres /usr/lib/postgresql/11/bin/initdb /data/
[INFO]
[INFO] sudo: /usr/lib/postgresql/11/bin/initdb: command not found
[INFO]
[ERROR] The command '/bin/sh -c /pulsar/django/init-postgres.sh' returned a non-zero code: 1
[WARNING] An attempt failed, will retry 1 more times
org.apache.maven.plugin.MojoExecutionException: Could not build image
    at com.spotify.plugin.dockerfile.BuildMojo.buildImage (BuildMojo.java:247)
    at com.spotify.plugin.dockerfile.BuildMojo.execute (BuildMojo.java:135)
    at com.spotify.plugin.dockerfile.AbstractDockerMojo.tryExecute (AbstractDockerMojo.java:265)
    at com.spotify.plugin.dockerfile.AbstractDockerMojo.execute (AbstractDockerMojo.java:254)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:956)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:288)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:192)
    at sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:498)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)
The root cause is when we use ubuntu 20.04 use base docker image instead of openjdk:11-jdk image introduce by #11026 , the ubuntu 20.04 will install postgresql 12 instead of postgresql 11 by default. However, the init and start script has been hard code with postgresql 11 install path, which will lead to command not found exception.

### Modification
1. add postgresql 11 resource address for ubuntu 20.04 and install postgresql 11

docker/pulsar-standalone/Dockerfile

@@ -28,16 +28,20 @@ FROM ubuntu:20.04
 
 ARG DEBIAN_FRONTEND=noninteractive
 
+RUN apt-get update \
+    && apt-get -y install wget gnupg && wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -
+RUN bash -c "echo deb http://apt.postgresql.org/pub/repos/apt/ focal-pgdg main >> /etc/apt/sources.list.d/pgdg.list"
+
 # Note that the libpq-dev package is needed here in order to install
 # the required python psycopg2 package (for postgresql) later
 RUN apt-get update \
-    && apt-get -y install openjdk-11-jdk-headless python3 python3-dev python3-pip postgresql sudo nginx supervisor libpq-dev
+    && apt-get -y install openjdk-11-jdk-headless python3 python3-dev python3-pip postgresql-11 sudo nginx supervisor libpq-dev
 
 RUN update-alternatives --install /usr/bin/python python /usr/bin/python3 10
 RUN update-ca-certificates
 
 # Postgres configuration
-COPY --from=dashboard /etc/postgresql/11/main/postgresql.conf /etc/postgresql/12/main/postgresql.conf
+COPY --from=dashboard /etc/postgresql/11/main/postgresql.conf /etc/postgresql/11/main/postgresql.conf
 
 # Configure supervisor
 COPY --from=dashboard /etc/supervisor/conf.d/supervisor-app.conf /etc/supervisor/conf.d/supervisor-app.conf