Describe the bug
Dockerfile syntax highlighting is incorrect for continuations including comments.

To Reproduce
vim Dockerfile

Example Dockerfile:

FROM debian:10.3

RUN apt-get update -y \
    && apt-get upgrade -y \
    && apt-get install -y curl grep sed unzip git sudo jq gettext \
    # Azure CLI
    && cd /tmp \
    && curl -sL https://aka.ms/InstallAzureCLIDeb | bash \
    # Clean-up Apt Caches
    && apt-get clean \
    && rm -rf /var/lib/apt/lists

Expected behavior
The comments shouldn't count towards the highlight behaviour of continuation lines in RUN as Docker ignores comments for the purposes of continuing lines.

The above script works when using docker build . and so the highlighting should also show this.

Screenshots

Environment (please complete the following information):
I'm not sure the environment matters with this but here it is:

• Vim version [e.g. 8.1.1234] (Or paste the result of vim --version.)
  • VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Mar 30 2020 21:58:19)
    Included patches: 1-486
    Modified by cygwin@cygwin.com
    Compiled by cygwin@cygwin.com
• OS: Windows 10
• Terminal: mintty