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

.build-deps is an arbitrary name to call a "virtual package" in Alpine, where you will add packages.

It creates an extra 'world' of packages, that you will need for a limited period of time (e.g. compilers for building other things).

Its main purpose is to keep your image as lean and light as possible, because you can easily get rid of it once those packages were used.

Please remember that it should be included in the same RUN if you want to achieve the main purpose of lightweight.

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

RUN apk add --virtual mypacks gcc vim \
 && apk del mypacks

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

In docker these must be executed as a single RUN command (as shown above), otherwise it will not reduce the image size.