Fix beginners guide docker instruction

[knelli2]

Since we changed the demo container so that the entry point is the
CLI, this broke the instructions on the tutorial page. Adding
--entrypoint "/bin/bash" fixes this.

Proposed changes

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[MarloMo]

Looks good to me, thank you for the fix!