Add Syntax Support

[vguaglione]

We are starting to make use of heredoc syntax in our dockerfiles and are requesting the addition of this support in the kaniko container.

A quick test revealed the executor can not parse the syntax in the dockerfile. Below is a sample dockerfile that implements the syntax and the resulting error by the executor:

# syntax=docker/dockerfile:1-3-labs
FROM bitnami/php-fpm:latest

EXPOSE 9000
WORKDIR /app

RUN <<EOF
apt-get update -y
apt-get install -y php-mbstring php-mysqli
EOF

error building image: parsing dockerfile: Dockerfile parse error line 15: unknown instruction: APT-GET