How to use Azure variable $(build.buildnumber)-$(build.sourcebranchname) in Dockerfile as build artifact/jar is getting generated with buildNumber?

I am writing a azure pipeline that will build a jar and a docker image. Most of part is working but I am not able to substitute $version variable in Dockerfile. And I am declaring artifact version as $(build.buildnumber)-$(build.sourcebranchname) as given below. I can not hard code it as with every run of build, builnumber will change. How can I use either $version or $(build.buildnumber)-$(build.sourcebranchname) in Dcokerfile where I am copying jar to container folder.

My generated artifact name is like this someservice-api-$(build.buildnumber)-$(build.sourcebranchname).jar

Azure Pipeline:

pool:
  vmImage: 'windows-latest'

steps:

- task: DownloadSecureFile@1
  displayName: 'Download gradle.properties secure file'
  inputs:
    secureFile: gradle.properties

- powershell: |
    New-Item -Type Directory -Force "${HOME}/.gradle"
    $version = "$(build.buildnumber)-$(build.sourcebranchname)"
    Add-Content "$(Agent.WorkFolder)/_temp/gradle.properties" "`nartifactVersion=$version"
    Add-Content "$(Agent.WorkFolder)/_temp/gradle.properties" "`nartifactDir=$path"
    Copy-Item -Force "$(Agent.WorkFolder)/_temp/gradle.properties" "${HOME}/.gradle/gradle.properties"

other task like gradlew build, copy, publish to staging directory

Dockerfile:

FROM openjdk:8
WORKDIR /usr/src/webapp/clientscore
COPY ./build/libs/someservice-api-$version.jar api.jar
CMD ["java", "-jar", "api.jar"]

I have tried with following already

COPY ./build/libs/someservice-api-$version.jar api.jar
COPY ./build/libs/someservice-api-${version}.jar api.jar
COPY ./build/libs/someservice-api-$(build.buildnumber)-$(build.sourcebranchname).jar api.jar
COPY ./build/libs/someservice-api-${build.buildnumber}-${build.sourcebranchname}.jar api.jar

Azure Docker task:

- task: Docker@2
  inputs:
    command: 'buildAndPush'
    Dockerfile: '**/Dockerfile'