How to extract a .dmg file in Linux?

Just use 7z x.

In the case of for example Sublime text, 7z x "Sublime Text 2.0.2.dmg" will be enough to extract all the files.

In other cases, like for example the JDK, you have to deal with some kind of matryoshka.

$ 7z x jdk-8u51-macosx-x64.dmg
$ cd JDK 8 Update 51/
$ 7z x JDK 8 Update 51.pkg
$ 7z x Payload~

But eventually you will get a folder containing the files you're looking for.