How do you assign the output of a command to the ARG variable in a Dockerfile?

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To assign the output of a command to the ARG variable in a Dockerfile, you can use the following syntax:

ARG VARIABLE_NAME=$(command)

For example, if you want to assign the output of the date command to an ARG variable called BUILD_DATE, you would use:

ARG BUILD_DATE=$(date +%Y-%m-%d)

This will set the value of the BUILD_DATE ARG variable to the current date in YYYY-MM-DD format. You can then use this variable in other parts of your Dockerfile, such as in labels or ENV statements.