Add explicit host variable setup for compose without links

• Allows explicitly setting variables following the pattern <service_name>_HOST and <service_name>_PORT
• The -legacy flag allows usage for the links environment with automatically set environment variables

v0.5 Updated golang to 1.11.2

This update only includes the golang update from 1.5 to 1.11.2.

Alpine Linux

This adds support for Alpine Linux by building waitforservices with CGO_ENABLED=0.

Thanks, @SerialVelocity!

0.3 Deduplication

Previously, if the same service (same IP and port) appeared in
multiple environment variables, waitforservices did the health check
for each environment variable, possibly multiple times per service.

Now, deduplicate services by IP and port and choose the one with the
shortest name.

There's an attached binary for x86-64 Linux for installation into a Docker image (see README).

0.2 Log wait duration

There's an attached binary for x86-64 Linux for installation into a Docker image (see README).

0.1 - Initial release

There's an attached binary for x86-64 Linux for installation into a Docker image (see README).