[bash] Update Bash Client Dockerfile.mustache

[avbenavides]

Using alpine instead of ubuntu for leaner Docker images.

Small fix for welcome message.

Changing default shell to zsh and fixing entrypoint.

@bkryza @frol @bkryza @kenjones-cisco

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

This PR targets 6.0.x, which has breaking changes without fallbacks.

Can this change target master instead? or can you elaborate on the breaking changes without fallbacks?

[avbenavides]

@wing328 Targeting master

[wing328]

I've filed #9310 instead to address the merge conflicts