Update npm in the Dockerfile, combine RUN commands

[cmenon12]

Description

Update npm to in the Dockerfile. Combine the RUN commands to reduce layers.

This follows on from #91.

Related issues: none

Checklist before marking as ready for review

Before marking as ready for review, please check that you have:

• Added appropriate and passing tests for the changes I made.
• Not broken any existing functionality or tests.
• Not introduced any new linting or formatting errors.
• Only added new dependencies that are necessary, do not introduce security vulnerabilities, and with tilde (~) version ranges.
• Checked and resolved UI accessibility issues using Axe devtools. Unresolved issues are described above.
• Updated the documentation if necessary.

Check the contributing guidelines for more details.

Screenshots (if appropriate)