composer dockerized

Smaller composer image, based on newer package versions.

tico/composer vs composer

• Used alpine:edge instead of php:7-alpine3.7 as base image
• Removed following dependencies: bash, mercurial, subversion, openssh, openssl (php ext works with libopenssl, not openssl itself), zlib-dev (php ext installs it automatically), curl (php ext uses libcurl, not curl itself) and many more default unused packages from official php image
• Refactored Dockerfile
• Reduced image size