### Problem
Docker builds started failing with a misleading dependency resolution
error:
```
Could not find compatible versions
Because every version of devise-secure_password depends on railties >= 5.0.0, < 8.0.0
...
```
### Root Cause
[Bundler
4.0.4](https://github.com/ruby/rubygems/releases/tag/bundler-v4.0.4) was
released on January 14, 2026. The Dockerfile used `gem install bundler`
without version pinning, which installed the latest (4.0.4) instead of
the version specified in `Gemfile.lock` (2.5.16).
### Fix
- Pin Bundler installation to match `Gemfile.lock`: `gem install bundler
-v "$BUNDLER_VERSION"`
- Update `BUNDLER_VERSION` from 2.5.11 to 2.5.16
### ⚠️ Note
Fix found by Claude while checking why the deploy was failing, and have
not yet checked for alternatives which might be more appropriate, so
further investigation might be required.
da42a4e4a1