Add Alpine Linux

[tom-tan]

This request fixes #2.

It build alpine images for dmd, ldc, and gdc.
It does nothing for non-stable versions (e.g., dmd-beta, ldc-beta).

I confirmed that the following commands work in the resulted images as intended.

For dmd:

$ dmd sample.d; ./sample
$ rdmd --compiler=dmd sample.d
$ dub init foo; cd foo; dub run # Note: initialize the directory with default options

For ldc:

$ ldc2 sample.d; ./sample
$ ldc2 sample.d -static; ./sample # for static linked binary
$ rdmd sample.d
$ dub init foo; cd foo; dub run

For gdc:

$ gdc sample.d -o sample; ./sample
$ gdc sample.d -o sample -static; ./sample # for static linked binary
$ rdmd --compiler=gdmd sample.d # Note: it prints annoying log messages but solving it is out of scope of this request
$ dub init foo; cd foo; dub run

sample.d (used in the above examples):

import std.stdio;

void main()
{
    writeln("hello");
}

Note:
It uses the official packages provided by Alpine Linux instead of using packages by dlang.org as used in other Linux distributions.
Therefore it cannot use D_VERSION_RESOLVED because Alpine Linux has different release cycles for D compilers.
Instead of that, it introduces D_VERSION_RESOLVED_ALPINE that specifies the latest official package version without package revision number (See "show_resolved_version.sh" for details).

[tom-tan]

Ping!