fix: install gdb in Docker container

[jdblischak]

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While I was going through episode 8, I discovered that the Docker container didn't have gdb installed

Note that I got the syntax for linking between files from the lesson-example here (I also confirmed on the site published from my fork that it works)

[jdblischak]

By the way, thanks for creating and sharing this great tutorial on CMake. Much appreciated!

[henryiii]

And thanks for helping!