Fail the devcontainer CI job when rake spec fails

`.github/workflows/devcontainer.yml`'s `runCmd` is a multi-line bash
script with no `set -e`, so a non-zero exit from
`bundle exec rake spec` does not propagate. The script keeps running,
executes `bundle exec rubocop`, and the script's exit code becomes
RuboCop's exit code. As long as RuboCop is clean, GitHub Actions
marks the job green even when rspec is red.

Add `set -e` so the first non-zero exit aborts the script and surfaces
through `devcontainers/ci` as a job failure. Mirrors
rsim/oracle-enhanced#2801.

Scope kept intentionally minimal: one `set -e` line. `-u` risks
tripping on unset-but-harmless variables and the immediate bug is
just exit-code propagation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

.github/workflows/devcontainer.yml

@@ -18,6 +18,7 @@ jobs:
         uses: devcontainers/ci@v0.3
         with:
           runCmd: |
+            set -e
             uname -a
             lsb_release -a
             ruby --version