Describe the bug
The new feature - #57 SSH socket sharing - doesn't seem to work when we have to deal with private repositories. When I try to run composer require on a private repository inside the php container, my public key is not recognized while it is working outside the container.

To Reproduce
Steps to reproduce the behavior:

1. Go inside thee php container
2. Try to add private repository as a dependency
3. See error

Expected behavior
The dependency should be cloned without problem.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: MacOS
• Version 10.15.1
• Docker version: Edge 2.1.6.0

Note: The issue can be reproduced both with and without passphrase on the SSH key.

Main Error:
E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation

During the PHP build:
`Libraries have been installed in:
/usr/src/php/ext/zip/modules
If you ever happen to want to link against installed libraries
in a given directory, LIBDIR, you must either use libtool, and
specify the full pathname of the library, or use the '-LLIBDIR'
flag during linking and do at least one of the following:

• add LIBDIR to the 'LD_LIBRARY_PATH' environment variable
  during execution
• add LIBDIR to the 'LD_RUN_PATH' environment variable
  during linking
• use the '-Wl,--rpath -Wl,LIBDIR' linker flag
• have your system administrator add LIBDIR to '/etc/ld.so.conf'

See any operating system documentation about shared libraries for
more information, such as the ld(1) and ld.so(8) manual pages.

Build complete.
Don't forget to run 'make test'.

Installing shared extensions: /usr/local/lib/php/extensions/no-debug-non-zts-20160303/
find . -name *.gcno -o -name *.gcda | xargs rm -f
find . -name *.lo -o -name *.o | xargs rm -f
find . -name *.la -o -name *.a | xargs rm -f
find . -name *.so | xargs rm -f
find . -name .libs -a -type d|xargs rm -rf
rm -f libphp.la modules/* libs/*

Installing the NodeSource Node.js 9.x repo...

Populating apt-get cache...

apt-get update
Hit:1 http://security.debian.org/debian-security stretch/updates InRelease
Ign:2 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Hit:3 http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease
Hit:4 http://cdn-fastly.deb.debian.org/debian stretch Release
Reading package lists...

Installing packages required for setup: apt-transport-https lsb-release...

apt-get install -y apt-transport-https lsb-release > /dev/null 2>&1

Confirming "stretch" is supported...

curl -sLf -o /dev/null 'https://deb.nodesource.com/node_9.x/dists/stretch/Release'

Adding the NodeSource signing key to your keyring...

curl -s https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add -
E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation
Error executing command, exiting
ERROR: Service 'php' failed to build: The command '/bin/sh -c apt-get update -qq && apt-get install -y --no-install-recommends bzip2 git libcurl4-openssl-dev libicu-dev libfreetype6-dev libjpeg-dev libmcrypt-dev libpng-dev libxml2-dev libxslt-dev ssmtp && docker-php-ext-configure bcmath && docker-php-ext-configure gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/ && docker-php-ext-install -j$(nproc) bcmath gd intl mcrypt opcache pdo_mysql soap xsl zip && perl -pi -e "s/mailhub=mail/mailhub=maildev/" /etc/ssmtp/ssmtp.conf && usermod -u 1000 www-data && curl -sS https://deb.nodesource.com/setup_9.x | bash && apt-get install -y --no-install-recommends nodejs && npm install -g grunt-cli && rm -rf /var/lib/apt/lists/*' returned a non-zero code: 1`