Mention Docker image in INSTALLATION.md
Mention Docker image in INSTALLATION.md

exercism/xperl6,mienaikage/exercism-perl6,exercism/xperl6,mienaikage/xperl6,mienaikage/xperl6,mienaikage/xperl6,exercism/xperl6,mienaikage/exercism-perl6,mienaikage/exercism-perl6