Add "container.appRoot" config parameter

[chanseokoh]

The default will be /app, so when not explicitly given, the normal non-WAR Jib builds will work as before.

Knowing that users can already configure ENTRYPOINT, this new config parameter will help people use an arbitrary Servlet engine as a base image for building a WAR image (#431). In the WAR case, the default value will be /jetty/webapps/ROOT. (The default base image will be gcr.io/distroless/java/jetty.)

Don't forget to add docs after releasing it.

[coollog]

Alternative naming suggestion: container.app.root or container.appRoot

[chanseokoh]

container.appRoot sounds good. .app.root looks like two levels.

[loosebazooka]

I like appRoot as the param name. Can you show how it will look in gradle/maven config?

[chanseokoh]

I imagine this is under <container>, specifying a single directory as a string type.

  jib.container.appRoot = '/app'

  <configuration>
    <container>
      <appRoot>/app</appRoot>
    </container>
  </configuration>

[chanseokoh]

Should add a doc.

[TadCordle]

Added this to the README (reopen if you meant something else)