Please use the combined Dockerfile and docker-compose Filetype
Dockerfiles and docker-compose filetypes are now merged into one filetype. Therefore this filetype for only Dockerfiles files will not be kept updated.
Go to the filetype Dockerfile and docker-compose Filetype instead.

Dockerfile is a file format used to build Docker images.

Docker is a computer program that performs operating-system-level virtualization, also known as “containerization”

This page describe how to add basic Dockerfile syntax highlighting on Geany.

Add the following lines to ~/.config/geany/filetype_extensions.conf under the respective sections [Extensions] and [Groups]

[Extensions]
Dockerfile=Dockerfile;dockerfile;*.dockerfile;*.Dockerfile;
[Groups]
Script=Dockerfile;

Save the configuration file below in your Geany filetype definition files config directory, e.g. ~/.config/geany/filedefs/filetypes.Dockerfile.conf

filetypes.Dockerfile.conf

# For complete documentation of this file, please see Geany's main documentation
#
# Geany Color Syntax support for Dockerfiles 
#
# version 0.2
#
# 0.1 -  original version
# 0.2 -  base on Sh instead - fixes problems with //, #, /* 
 
[styling=C]
 
[keywords]
primary= ADD ARG CMD COPY ENTRYPOINT ENV EXPOSE FROM HEALTHCHECK LABEL ONBUILD RUN SHELL STOPSIGNAL USER VOLUME WORKDIR
 
[lexer_properties=C]
 
[settings]
lexer_filetype=Sh
tag_parser=C
extension=
# single comments, like # in this file
comment_single=#
 
# set to false if a comment character/string should start at column 0 of a line, true uses any
# indentation of the line, e.g. setting to true causes the following on pressing CTRL+d
	#command_example();
# setting to false would generate this
#	command_example();
# This setting works only for single line comments
comment_use_indent=true
# context action command (please see Geany's main documentation for details)
context_action_cmd=xdg-open "https://docs.docker.com/engine/reference/builder/"