Is there a short 7-digit version of $(SourceVersion) in Azure Devops?

You can use traditional command substitution via backticks to obtain the short git hash (SHA-1), assuming that the code is being checked out in $(Build.SourcesDirectory):

  - bash: |
      short_hash=`git rev-parse --short=7 HEAD`  ## At least 7 digits, more if needed for uniqueness
      echo ""
      echo "Full git hash:  $(Build.SourceVersion)"
      echo "Short git hash: $short_hash"
      echo "##vso[task.setvariable variable=short_hash]$short_hash"  ## Store variable for subsequent steps
    workingDirectory: $(Build.SourcesDirectory)
    displayName: Get short git hash

Output:

Full git hash:  f8d63b1aaa20cf348a9b5fc6477ac80ed23d5ca0
Short git hash: f8d63b1

The following steps in the pipeline can then use the short hash via the variable $(short_hash).

(This is better than manually trimming down the full git hash to seven characters, since this will add extra digits if needed to uniquely identify the commit, see https://stackoverflow.com/a/21015031/1447415.)

Update: Improved version

The following improved version checks that the git hashes match (that the full hash starts with the short hash) and fails the step otherwise:

  - bash: |
      short_hash=`git rev-parse --short=7 HEAD`
      echo ""
      echo "Full git hash:  $(Build.SourceVersion)"
      echo "Short git hash: $short_hash"
      echo ""
      ## Fail step if full hash does not start with short hash
      if [[ $(Build.SourceVersion) != $short_hash* ]]; then
        echo "--> Hashes do not match! Aborting."
        exit 1
      fi
      echo "--> Hashes match. Storing short hash for subsequent steps."
      ## Store variable for subsequent steps
      echo "##vso[task.setvariable variable=short_hash]$short_hash"
    workingDirectory: $(Build.SourcesDirectory)
    displayName: Get short git hash

- script: |
    echo $sourceVersion
    commitHash=${sourceVersion:0:7}
    echo $commitHash
    echo "##vso[task.setvariable variable=commitHash]$commitHash" ## Set variable for using in other tasks.
  env: { sourceVersion: $(Build.SourceVersion) }
  displayName: Git Hash 7-digit
  workingDirectory: #workingDirectory

- task: Docker@2
  displayName: Build & Push image
  inputs:
    command: 'buildAndPush'
    containerRegistry: '$(myRegistry)'
    repository: $(myContainerRepository)
    Dockerfile: $(myDockerfile)
    buildContext: '$(myBuildContext)'
    tags: $(commitHash) ## The variable was defined above.

Here's example for vmImage: "ubuntu-latest". Step:

1. Split 7-characters from Pre-defined GitHash
2. Assign it to Pipeline variable. Don't confuse $(azure_pipeline_variable) with ${bash_shell_variable} or $bash_shell_variable.
3. Use it by $(commitHash)

Read more:

1. Assign variable in Azure pipeline
2. Using script in Azure pipeline