ci: fixed bundler version to avoid bundler 4.0.4 incompatibility (#13290)

### 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.

docker/Dockerfile

@@ -11,7 +11,7 @@ ENV PNPM_VERSION=${PNPM_VERSION}
 # For development docker-compose file overrides ARGS
 ARG BUNDLE_WITHOUT="development:test"
 ENV BUNDLE_WITHOUT ${BUNDLE_WITHOUT}
-ENV BUNDLER_VERSION=2.5.11
+ENV BUNDLER_VERSION=2.5.16
 
 ARG RAILS_SERVE_STATIC_FILES=true
 ENV RAILS_SERVE_STATIC_FILES ${RAILS_SERVE_STATIC_FILES}
@@ -35,7 +35,7 @@ RUN apk update && apk add --no-cache \
   curl \
   xz \
   && mkdir -p /var/app \
-  && gem install bundler
+  && gem install bundler -v "$BUNDLER_VERSION"
 
 COPY --from=node /usr/local/bin/node /usr/local/bin/
 COPY --from=node /usr/local/lib/node_modules /usr/local/lib/node_modules
@@ -105,7 +105,7 @@ ENV PNPM_VERSION=${PNPM_VERSION}
 
 ARG BUNDLE_WITHOUT="development:test"
 ENV BUNDLE_WITHOUT ${BUNDLE_WITHOUT}
-ENV BUNDLER_VERSION=2.5.11
+ENV BUNDLER_VERSION=2.5.16
 
 ARG EXECJS_RUNTIME="Disabled"
 ENV EXECJS_RUNTIME ${EXECJS_RUNTIME}
@@ -128,7 +128,7 @@ RUN apk update && apk add --no-cache \
   imagemagick \
   git \
   vips \
-  && gem install bundler
+  && gem install bundler -v "$BUNDLER_VERSION"
 
 COPY --from=node /usr/local/bin/node /usr/local/bin/
 COPY --from=node /usr/local/lib/node_modules /usr/local/lib/node_modules