#Dockerfile reference – @Docker – Slash \  is an escape char at the of a line 

This is why my Docker builds were dying and telling me stuff about incorrect sytax and commands. I put a \ Slash at the end of the line.

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 Wi

Source: Dockerfile reference – Docker