docker - What is .build-deps for apk add --virtual command?

Question

What is .build-deps in the following command? I can't find an explanation in the Alpine docs. Is this a file that is predefined? Is see this referenced in many Dockerfiles.

RUN apk add --no-cache --virtual .build-deps 
gcc 
freetype-dev 
musl-dev

RUN pip install --no-cache-dir <packages_that_require_gcc...> 

RUN apk del .build-deps

Answer 1

If you see the documentation

  -t, --virtual NAME    Instead of adding all the packages to 'world', create a new 
                        virtual package with the listed dependencies and add that 
                        to 'world'; the actions of the command are easily reverted 
                        by deleting the virtual package

What that means is when you install packages, those packages are not added to global packages. And this change can be easily reverted. So if I need gcc to compile a program, but once the program is compiled I no more need gcc.

I can install gcc, and other required packages in a virtual package and all of its dependencies and everything can be removed this virtual package name. Below is an example usage

apk add --virtual mypacks gcc vim
apk del mypacks

The next command will delete all 18 packages installed with the first command.