Multi-stage builds separate compilation from the runtime image. Copy only artifacts and dependencies actually needed to run.
Before you start
You should understand the difference between an image, a running container and the host. Record where a file, process or network endpoint actually lives before diagnosing a problem. Commands illustrate local experiments; adapt image names and paths to a disposable development environment.
The practical goal is to reason through this situation: A builder includes compilers while the final image contains the application and runtime. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Separate build and runtime
The compiler stage needs tools the running application may not need.
Step 2: Copy selected artifacts
Transfer the application output and required runtime dependencies only.
Step 3: Test the final environment
Check libraries, permissions and paths in the final stage.
Worked scenario
A builder includes compilers while the final image contains the application and runtime.
A builder compiles a binary with native dependencies. Copying only the binary into an incompatible runtime image may fail even though compilation succeeded. Determine its runtime libraries and architecture, then test the produced image rather than treating a small final stage as automatic completeness.
Common mistake
Copying the whole builder filesystem defeats the separation.
Verify the behavior
Run the final image without builder files and exercise startup plus one meaningful operation.
Interview exercise
Verify the final stage.
Answer and reasoning
Run the artifact in that stage and inspect required files, permissions and native-library dependencies.
Continue learning
Compare the scenario with the Docker interview questions and test your understanding with the Docker MCQs. For terminology and implementation details, consult the reference material.