What does the ENTRYPOINT in a Dockerfile do?

What does the ENTRYPOINT in a Dockerfile do?

• A. Specifies the command to run when the container is started
• B. Creates a new user inside the image
• C. Installs software packages at build time
• D. Sets environment variables for the container

Answer: (A)

The startup command that runs when the container starts is defined by ENTRYPOINT. It fixes what executable is launched every time a container is started from this image, ensuring predictable behavior. For example, setting the entrypoint to a specific application makes the container behave like a dedicated service or tool, starting that app automatically with any provided arguments. Using the exec form, such as ["path/to/app", "arg1", "arg2"], ensures the process receives signals properly and avoids shell interpretation, which is important for clean shutdowns and correct PID handling. The arguments you pass after the image, either via CMD or the docker run command, are treated as arguments to this entrypoint, but the entrypoint itself remains the fixed starting point. Other Dockerfile instructions do different jobs: USER creates the user context inside the image, RUN installs software at build time, and ENV sets environment variables. This separation helps you design images where the startup behavior is reliable (via ENTRYPOINT) while configuration and environment can be provided separately. If you ever need to override the entire command at runtime, you can adjust it with docker run options, but by default the entrypoint is what the container uses to start.

The startup command that runs when the container starts is defined by ENTRYPOINT. It fixes what executable is launched every time a container is started from this image, ensuring predictable behavior. For example, setting the entrypoint to a specific application makes the container behave like a dedicated service or tool, starting that app automatically with any provided arguments. Using the exec form, such as ["path/to/app", "arg1", "arg2"], ensures the process receives signals properly and avoids shell interpretation, which is important for clean shutdowns and correct PID handling. The arguments you pass after the image, either via CMD or the docker run command, are treated as arguments to this entrypoint, but the entrypoint itself remains the fixed starting point. Other Dockerfile instructions do different jobs: USER creates the user context inside the image, RUN installs software at build time, and ENV sets environment variables. This separation helps you design images where the startup behavior is reliable (via ENTRYPOINT) while configuration and environment can be provided separately. If you ever need to override the entire command at runtime, you can adjust it with docker run options, but by default the entrypoint is what the container uses to start.