The bombadier beetle uses a hot, chemical spray to ward off predators without hurting itself - if a dragon had similar glands under its scales, it could raise its scales and release a cloud around it of boiling, noxious gas that would burn anything next to it. It wouldn't light up though.
In order to get flames, we either need sparks, a stronger exothermic reaction, or something that ignites at lower temperatures.
If we keep the glands under the scales, but fill them with oil or alcohol - something easy to produce and flammable - and then give the dragon some way to ignite it with a spark when sprayed from under the scales.
Hydrogen gas is an option, too - it could be generated from the dragon's stomach acids.
Since dragon scales are famously hard, iron pyrite might do the trick; if you want something a little more exotic, then the eletricity-generating organ similar to electric eels or electric rays are able to deliver enough voltage to generate a spark - they do not spark in those animals, the dragon would need specialised conducting elements to generate a spark.
Something like sodium peroxide + zinc + water will produce a reaction strong enough to burn, but due to the sodium hydroxide being strongly reactive with water it will be difficult to produce or contain inside an animal. The other highly reactive metals (lithium, potassium) have similar problems.
Low Temperature Ignition
Carbon disulphide has an autoignition temperature of only 90 degrees celcius - a temperature that can be reached by the bombadier beetle's chemical mix. The carbon disulphide could be sprayed around the dragon, then ignited with a single squirt.
The problem is that carbon disulphide is highly toxic and a strong solvent, so it would be impossible for the dragon to store. It would also be impossible to manufacture - most processes for creating it involve temperatures above 600 degrees - so the dragon would need to find a source (e.g. volcanoes), and could not produce it on demand.
Less Violent Reactions
Most of the above prioritise reaction speed, so the dragon can use it as a defence mechanism on demand. There are some other processes that might be able to be controlled appropriately or hand-waved away so cover the dragon in flames. A good example is pistachios, which will, all by themselves, catch fire and burn if enough are stored together in a humid environment. If a dragon were to have a pistachio-shell-like compound in a special organ near its skin, where it can control the humidity and oxygen (and thus, the temperature build-up), it may be able to have a reservoir of burning material it can either expel or use to power one of the processes above.