diff --git a/Dockerfile b/Dockerfile
index 27df589..31f2f75 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,9 +1,11 @@
-FROM ruby:2.6.5-buster AS development
+FROM ruby:2.6.5-alpine AS development
 
 WORKDIR /usr/src
 
 ENV HOME=/usr/src PATH=/usr/src/bin:$PATH
 
+RUN apk add --no-cache su-exec alpine-sdk
+
 COPY Gemfile Gemfile.lock on_container.gemspec /usr/src/
 COPY lib/on_container/version.rb /usr/src/lib/on_container/
 
@@ -15,7 +17,7 @@ ARG DEVELOPER_USERNAME=you
 
 ENV DEVELOPER_UID=${DEVELOPER_UID}
 
-RUN useradd -r -M -u ${DEVELOPER_UID} -d /usr/src -c "Developer User,,," ${DEVELOPER_USERNAME}
+RUN adduser -D -H -u ${DEVELOPER_UID} -h /usr/src -g "Developer User,,," ${DEVELOPER_USERNAME}
 
 FROM development AS testing
 
diff --git a/Gemfile b/Gemfile
index c02d70d..0608195 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,3 +5,6 @@ gemspec
 
 gem "rake", "~> 12.0"
 gem "rspec", "~> 3.0"
+
+gem 'byebug'
+gem 'pry-byebug'
\ No newline at end of file
diff --git a/Gemfile.lock b/Gemfile.lock
index 695f479..ede15b3 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -6,7 +6,16 @@ PATH
 GEM
   remote: https://rubygems.org/
   specs:
+    byebug (11.1.1)
+    coderay (1.1.2)
     diff-lcs (1.3)
+    method_source (0.9.2)
+    pry (0.12.2)
+      coderay (~> 1.1.0)
+      method_source (~> 0.9.0)
+    pry-byebug (3.8.0)
+      byebug (~> 11.0)
+      pry (~> 0.10)
     rake (12.3.3)
     rspec (3.9.0)
       rspec-core (~> 3.9.0)
@@ -27,7 +36,9 @@ PLATFORMS
 
 DEPENDENCIES
   bundler (~> 1.17)
+  byebug
   on_container!
+  pry-byebug
   rake (~> 12.0)
   rspec (~> 3.0)
 
diff --git a/lib/on_container/step_down_from_root.rb b/lib/on_container/step_down_from_root.rb
index 96ad73c..4f88512 100644
--- a/lib/on_container/step_down_from_root.rb
+++ b/lib/on_container/step_down_from_root.rb
@@ -26,11 +26,11 @@ def root_user?
     end
 
     def developer_uid?
-      developer_uid_as_integer > 0
+      developer_uid > 0
     end
 
     def developer_uid
-      @developer_uid ||= ENV['DEVELOPER_UID'].to_i
+      @developer_uid ||= ENV.fetch('DEVELOPER_UID', '').to_i
     end
 
     protected
@@ -38,7 +38,7 @@ def developer_uid
     def switch_to_developer_user
       target_user_name = target_user.name
       puts "Switching from 'root' user to '#{target_user_name}'..."
-      exec 'su-exec', target_user_name, $0, *$*
+      Kernel.exec 'su-exec', target_user_name, $0, *$*
     end
 
     def warn_no_developer_uid
diff --git a/spec/step_down_from_root_spec.rb b/spec/step_down_from_root_spec.rb
new file mode 100644
index 0000000..adf8814
--- /dev/null
+++ b/spec/step_down_from_root_spec.rb
@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+require 'on_container/step_down_from_root'
+
+RSpec.describe OnContainer::StepDownFromRoot do
+  let(:root_user) do
+    instance_double "struct Etc::Passwd",
+                    name: 'root',
+                    passwd: 'x',
+                    uid: 0,
+                    gid: 0,
+                    gecos: 'root',
+                    dir: '/root',
+                    shell: '/bin/bash'
+  end
+
+  let(:developer_user) do
+    instance_double "struct Etc::Passwd",
+                    name: 'developer',
+                    passwd: 'x',
+                    uid: 1000,
+                    gid: 1000,
+                    gecos: 'Developer User,,,',
+                    dir: '/usr/src',
+                    shell: '/bin/bash'
+  end
+
+  let(:example_current_user) { root_user }
+  let(:example_target_user) { developer_user }
+  let(:example_developer_uid) { '1000' }
+
+  before do
+    allow(ENV).to receive(:fetch).with('DEVELOPER_UID', '') { example_developer_uid }
+    allow(Etc).to receive(:getpwuid) { example_current_user }
+    allow(Etc).to receive(:getpwuid).with(example_target_user.uid) { example_target_user }
+    allow(Kernel).to receive(:exec).with('su-exec', example_target_user.name, any_args)
+  end
+
+  describe '#perform' do
+    it 'changes to the target user' do
+      subject.perform
+      expect(Kernel).to have_received(:exec).with 'su-exec', example_target_user.name, any_args
+    end
+
+    context 'without a developer uid' do
+      let(:example_developer_uid) { '' }
+      
+      example 'does not change the current user' do
+        subject.perform
+        expect(Kernel).not_to have_received(:exec)
+      end
+    end
+
+    context 'when not as root' do
+      let(:example_current_user) { developer_user }
+      
+      example 'does not change the current user' do
+        subject.perform
+        expect(Kernel).not_to have_received(:exec)
+      end
+    end
+  end
+end