Update Docker base image to Node 16

[eyelidlessness]

I have verified this PR works with

• Online form submission
• Offline form submission
• Saving offline drafts
• Loading offline drafts
• Editing submissions
• Form preview
• None of the above

What else has been done to verify that this works as intended?

It builds, and appears to run (at least to the predictable point of failing to connect to Redis, which I believe is out of scope for this). The NPM version needed to be made explicit for build to succeed.

Why is this the best possible solution? Were any other approaches considered?

It's not. We should reevaluate what is an appropriate base image, and more explicitly manage dependencies and their versions—ideally at the project level, but at least in a consistent way (e.g. Volta for Node version management). And we should eliminate unneeded dependencies (e.g. Python).

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

The Docker image is using a different minor version of Node and NPM than the project, which could be a risk. But we're going to address the project level separately as part of enketo/enketo#951.

Do we need any specific form for testing your changes? If so, please attach one.

N/A.

[lognaturel]

Verified with Central. I wouldn't expect this to be risky given we've supported Node 16 for some time (aside from some possible unknowns around minor versions as highlighted above).

The way I see it, the short term goal is to get off the unmaintained node 14 base image to address known vulnerabilities. This achieves the goal. I do agree there are some deeper issues with the deployment examples maintained in this repo.