Add ubuntu-musl for ldc

[tom-tan]

This PR is to solve #2 partially by providing a way to build a single binary with dlang.

Preparing host compilers for Alpine Linux (i.e. with musl libc) is time consuming task because dmd, gdc and ldc do not provide a binary for it.

Instead of that, this request provides a way to build an ubuntu image with a cross compiler for musl libc.
It enables us to build a single binary with dlang and I guess it covers most use cases that use Alpine Linux.
This request only adds ldc-ubuntu-musl image because ldc provides ldc-build-runtime that easily builds runtime libraries for other architecture but other compilers do not.

ldc-ubuntu-musl image is built based on the dockerfile for ldc-ubuntu.
It sets -mtriple=x86_64-alpine-linux-musl and -static by default.
Sorry, I found that rdmd does not work if these options are the default setting.

root@da690b9721ba:/# echo 'import std; void main() { writeln("Hello!"); }' > sample.d
root@da690b9721ba:/# ldc2 -unittest -run sample.d # ldc2 works
Hello!
root@da690b9721ba:/# rdmd -unittest sample.d # but rdmd does not work
Aborted

To make less confusion, I make them as an option.

root@6de061b342bc:/# echo 'import std; void main() { writeln("Hello!"); }' > sample.d
root@6de061b342bc:/# ldc2 sample.d # compile with x86_64-unknown-linux-gnu by default
root@6de061b342bc:/# ldd sample
        linux-vdso.so.1 (0x00007fff4f1fe000)
        librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fca5efd0000)
        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fca5edcc000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fca5ebad000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fca5e80f000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fca5e5f7000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fca5e206000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fca5f1d8000)
root@6de061b342bc:/# ldc2 -mtriple=x86_64-alpine-linux-musl -static sample.d # specify to build a statically-linked binary with musl libc
root@6de061b342bc:/# ldd sample
        not a dynamic executable

Limitations:

• std.net.curl does not work because libcurl (installed via apt) is build with glibc
• It only supports -static (it is provided in the default settings) because I failed to prepare dynamic link libraries with musl libc(See a thread in gitter chat of ldc)

[wilzbach]

@tom-tan thanks a lot for your PR and I'm sorry that I missed this one.

Since a few weeks there's an ldc package in Alpine:

https://pkgs.alpinelinux.org/package/edge/testing/x86_64/ldc

Do you think we should change this PR to use this package instead?

[tom-tan]

@wilzbach Thank you for your reply!
IMO it is better to use Alpine package. It is easier to maintain and can build docker image faster than the current PR that builds cross compiler.

I will fix this PR to use Alpine package.

[tom-tan]

I close this request to investigate better solution using Alpine Linux instead of using tentative plan to use Ubuntu with musl libc.