using

The using instruction adds an additional, temporary image(s) for that container.

using <image>

Multiple images can be specified in a single using instruction by separating each image with a comma or space. If the same file, registry entry, or environment variable exists in multiple images added via the using instruction, then the one from whichever image was specified last will win the conflict and be used in the virtual environment. If the same file, registry entry, or environment variable exists in two images, one added via the using instruction and one added via the using instruction, then the one in the using image will win the conflict. Virtual machine settings are taken from the last image specified in the using instruction.

Due to this "layering" approach, it is a good practice to specify images with newer versions of applications or libraries after images with older versions.

If the next instruction after using is indented, a scope is defined and the images specified in the using instruction are available only in this scope.

using python
  cmd python --version

cmd python --version || echo Python not available!

Note: Images added with using keyword are NOT included in final image

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