fix(reaper): read RYUK_CONTAINER_IMAGE lazily so dotenv / runtime overrides work

[dvirarad]

Summary

Fixes #1310.

reaper.ts exports REAPER_IMAGE as a top-level const whose initializer reads process.env.RYUK_CONTAINER_IMAGE at module load time. As @reporter pointed out, that means anything that mutates the environment after the module is imported — most commonly a dotenv.config() call in user code, or an explicit process.env.RYUK_CONTAINER_IMAGE = ... in a test setup file — is silently ignored, and the user gets the default testcontainers/ryuk:0.14.0 image regardless.

This is inconsistent with every other RYUK_* / TESTCONTAINERS_RYUK_* env var in the same file (lines 36, 93, 100, 103, 106, 109…), all of which are read lazily inside createNewReaper().

Fix

Convert REAPER_IMAGE (eager const) → getReaperImage() (lazy function), matching the existing pattern. Two files touched:

• packages/testcontainers/src/reaper/reaper.ts — export becomes a function, call site at the bottom of createNewReaper uses getReaperImage().
• packages/testcontainers/src/generic-container/generic-container.ts — the two === REAPER_IMAGE comparisons become === getReaperImage().

The function is cheap (one env lookup + an ImageName.fromString parse), and both comparisons happen at construction / isReaper() check time — not in a hot loop — so the lazy evaluation has no measurable overhead.

API surface

REAPER_IMAGE was an exported symbol, so this is technically a breaking change for any external consumer that imports it directly. Two notes:

1. Public docs at docs/configuration.md and the README don't mention REAPER_IMAGE — only the env var RYUK_CONTAINER_IMAGE — so the export was effectively internal.
2. If maintainers prefer to keep the old name for back-compat, I can re-export getReaperImage as REAPER_IMAGE via an accessor (Object.defineProperty getter on the module's namespace), but that adds complexity for a name that wasn't documented. Happy to switch on request.

Test plan

• grep confirms no other REAPER_IMAGE references remain in the repo
• All other env-var reads in reaper.ts already use the lazy pattern; this change brings RYUK_CONTAINER_IMAGE into alignment
• CI green
• (optional follow-up) regression test in reaper.test.ts using vi.stubEnv("RYUK_CONTAINER_IMAGE", "...") + vi.resetModules() — happy to add if maintainers want it in this PR

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