diff --git a/Dockerfile b/Dockerfile
index 57857cd..974fac2 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,10 +1,11 @@
 FROM ruby:3.3
 ARG version
 
-RUN if [ -z ${version} ]; then                        \
+RUN gem update --system --no-document && \
+    if [ -z "$version" ]; then                        \
       gem install buildkite-builder;                  \
     else                                              \
-      gem install buildkite-builder -v ${version};    \
+      gem install buildkite-builder -v "$version";    \
     fi
 
-CMD buildkite-builder run
+CMD ["buildkite-builder", "run"]
diff --git a/VERSION b/VERSION
index 5c517bf..fb05571 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-4.15.0
+4.15.1
diff --git a/buildkite-builder.gemspec b/buildkite-builder.gemspec
index 2271e39..884fc73 100644
--- a/buildkite-builder.gemspec
+++ b/buildkite-builder.gemspec
@@ -29,5 +29,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake"
   spec.add_development_dependency "debug"
   spec.add_development_dependency "rspec"
+  spec.add_development_dependency "ostruct"
   spec.add_development_dependency "webmock"
 end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 1da7d83..c424685 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,6 +1,7 @@
 require 'bundler/setup'
 require 'buildkite-builder'
 require 'debug'
+require 'ostruct'
 require 'webmock/rspec'
 
 Dir[File.expand_path('support/**/*.rb', __dir__)].sort.each { |f| require_relative f }