Allow CGO_ENABLED=1

[e-nikolov]

Dynamically linked Go applications compiled with CGO_ENABLED=1 on a system with glibc (default on debian based distros) instead of musl (default on alpine) can now run inside an iron/go container.

[vasilev]

Thank you @e-nikolov for your contribution.

Could you provide an instruction how to test your changes ?

We tried to compile a simple example on Ubuntu using Go 1.5, and built a Docker image for that binary based on this Dockerfile.
We encountered the following error during image execution:

Error relocating /app/test: __vfprintf_chk: symbol not found
Error relocating /app/test: __fprintf_chk: symbol not found

Was this test correct ?

[e-nikolov]

Initially I tested only with a simple program that printed something to the screen and it worked, but after I started using CGO_ENABLED=1 features such as golang plugins, I got the same error as you. I ended up using https://github.com/frol/docker-alpine-glibc, which has a real glibc inside an alpine container, instead of trying to make musl look like glibc via symlinks.

The work around proposed here only seems to work when compiling a program with CGO_ENABLED=1, but not using features that require dynamic linking or linking with C programs, which is indeed not very useful.

In theory musl and glibc should be compatible and there should be a way to make CGO_ENABLED=1 work with just musl installed, but it seems that the proposed symlink is not sufficient.

Feel free to reject this pull request for now, if I come up with a better solution, I'll make another.