According to the official docker documentation
Everything that follows a # is considered as a comment inside of dockerfile.
# Everything on this line is a comment but it can be used only in the begining of the line
NOTE : There is something called a parser directive recently added and it shares it syntax with comments. So to differentiate between the both, parser directives need to be on top of the file before any commands or comments.
COPY testfile.txt c:\
RUN dir c:\
Hope this helps :)
Docker Training course can be very useful for anyone who is looking to get a job in the DevOps field.