add some runtime type-checking in Image.env

[thundergolfer]

Describe your changes

Action taken in response to user report: https://modal-com.slack.com/archives/C069RAH7X4M/p1739956895311459

But if feels a bit piecemeal? Don't really want to spread lots of runtime type-checking across the code just for the purposes of nicer error messages. Alternatively I've put up a docs change on the other repo (#20024)

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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[mwaskom]

I think these sort of runtime type assertions have to feel piecemeal in Python, but adding one here makes sense as this is an easy mistake to make, and the current error message is not very interpretable.

Would be great to have a small unit test for this!

[mwaskom]

Is there a reason you're deferring this till after catching a TypeError? One advantage to doing it immediately (i.e. not inside the build_dockerfile closure) is that the error will get attributed to the relevant line of user code; raising from the closure produces a very long and basically uninterpretable traceback.

I guess the downside is that code outside of the closure will execute on container cold starts which is undesirable. Shouldn't be too costly in most cases .... but every millisecond counts ;)