Linter does not follow ESCAPE directive

[lyricwulf]

As specified in Dockerfile reference:

# escape=\
# escape=`
The escape directive sets the character used to escape characters in a Dockerfile. If not specified, the default escape character is \.
The escape character is used both to escape characters in a line, and to escape a newline. This allows a Dockerfile instruction to span multiple lines. Note that regardless of whether the escape parser directive is included in a Dockerfile, escaping is not performed in a RUN command, except at the end of a line.
Setting the escape character to ` is especially useful on Windows, where \ is the directory path separator. ` is consistent with Windows PowerShell.

linter-docker does not currently check the escape directive and will report a line error if another character is used.