DOCKER ARG instruction as opposed to ENV instruction

 In Docker, ARG and ENV are used to define environment variables. The ARG instruction defines variables that users can pass to the builder at build-time. Unlike ENV (Environment Variables), values do not persist in the final image and are not available to the container once it is running.

Key Characteristics

• Build-Time Only: Variables defined with are only accessible during the image creation process (e.g., within or commands).
• Command Line Overrides: You can pass or override values using the flag with the command.
• Default Values: You can specify a default value in the Dockerfile (e.g., ) which is used if no value is passed during the build.
• Layer Visibility: While they don't persist at runtime, values are visible in the image's history via , so they should never be used for secrets like passwords or API keys.

Common Use Cases

• Version Management: Specifying versions for base images (e.g., ) or package installations.
• Build Customization: Enabling or disabling specific features or configurations based on the build environment (e.g., dev vs. prod).
• Metadata: Storing build-specific information like build dates or commit hashes as labels. 

Scoping Rules

• Stage Local: An is only available in the build stage where it is defined. In
  multi-stage builds
  
  , you must re-declare the in each stage if you need to use it there.
• Global Scope: An placed before the instruction is in the global scope and can be used to parameterize the command, but it must be re-declared after to be used in later instructions.