I think it is a passive voice. The sentence is:
Están pintando la puerta.
My question is why not
Está pintando la puerta.
It is not passive voice but the case is similar to impersonal passive voice.
Examples of impersonal passive are for example "It is said that children are afraid of ghosts" instead of the active sentence "People say that children are afraid of ghosts" or "It is expected (that) he will arrive soon" instead of "They expect him to arrive soon".
In Spanish, we favor plural (están) over singular (está) for this voice. Quoting @JuanRocamonde from the comments "plural form is used in substitution of an unknown subject".
Cuando se usa la tercera persona del plural de forma impersonal, es decir, que hay sujeto tácito de interpretación inespecífica (a veces llamadas 'oraciones impersonales eventuales'), el sujeto suele ser, en realidad, singular
Te llaman — Someone is calling you/You have a call
Te necesitan en la sala de espera — You're needed in the waiting room
Le robaron — He was robbed
Lo mataron — He was killed
Source: pasiva con tercera persona
This means that there is only one guy painting the door, but we use "plural", meaning "they (building management, or the contractor, or the painters, or who-knows-exactly-who-is-in-charge-of-it) are painting the door".
You could of course use the singular "está". The connotation is that you would likely know who is doing the job.
Están pintando la puerta --> (after switching to passive voice) The door is being painted. (by who? Who knows?)
Está pintando la puerta --> (after switching to passive voice) The door is being painted by him/her.
This is the same case as "they are doing construction here, they are cutting down this tree, etc". It is unclear who is exactly doing the job. We only know what is going to be done. In Spanish we would use plural for this case and use singular for the case in which we actually know who is doing the job (which probably can be inferred by the context of the conversation, since the sentence has an elliptical subject).
[This is a translation/update of my answer to another question.]
These are not passive sentences, but rather sentences that use an interesting impersonal construction, in fact the Spanish Grammar speaks about them in the paragraph 33.4s. Basically it says that when the subject of the sentence is unknown or is not identified, sometimes it adopts the form of the third person plural. So, you may have sentences such as
Mañana vendrán a reparar la caldera.
Están pintando la puerta.
In the first case you don't know indeed how many people will come. And in the second case you can use the third person plural even if there is only one person paiting because you have not indentify them. The funny thing is that you can use this construction even if you know the person implied:
Te han llamado por teléfono, era tu hermana.
The first part of the sentence follows this same pattern because you have not yet identified the person that called, even if you resolve that in the second part of the sentence.
This construction comes from long ago,in fact is as older as the Spanish language. See the following example from the XVI century:
Que aun hasta en lo que es música y en los cantares hallamos esto mismo, pues las seguidillas arrinconaron a la zarabanda y otros vendrán que las destruyan y caigan.
Mateo Alemán, "Primera parte de Guzmán de Alfarache", España (1599)
And this one from the XIII century:
Bien me as aconsejado, et dizes verdat; enpero veré a qué tornará la mi çima de mi fazienda et qué mandarán de mí fazer.
Anónimo, "Calila e Dimna", España (1251)
The last part of this last case can be translated as "let's see what they will tell me to do" or even as "let's see what I will be told to do". Note that in this last translation it is better reflected that you don't know how many people will be implied in the action. But the thing is that if you can find this construction in the XIII century, it probably comes from the Vulgar Latin.
You can find this construction in other languages, such as Italian:
"Ah, sei qui [...] finalmente ti trovo. Come ti senti?". "Un po' stanca ma non c'è male. Figurati, mi hanno telefonato". "Ti hanno telefonato? -ripete incredula- e chi?". "Una mia amica di Milano [...]".
Mirella Ducceschi, "L'incidente", Italia (1990)
This follows the same pattern as the previous example about stating the subject in third person plural even if the speaker knows who called.
As a final note, you can indeed say
Está pintando la puerta.
but then it's implied that you know who is painting the door. This can be said in response to
¿Dónde está Pepe?
Because "Están" is the plural third and second person conjugation for the verb "estar"
Remember that in Spanish we can remove the personal pronoun as the conjugation of the verb tells us who is the actor, so in normal speech we remove the "yo, tú, el/ella, nosotros, vosotros, ustedes, ellos/ellas" from sentences.
Also, this is not passive voice, this is normal continuous present:
Pronoun + "conjugation of present verb estar" + gerundio conjugation (ending with "ando/endo")
what you are trying to find with "está pintado la puerta" is an adverb (the verb of "pintar" as the adjective "pintado") , but you have a gender problem here, as adjective also have to inherit the genre of the noun:
correct : Está PINTADA la puerta, that means :
the door is painted.
This has a totally different meaning from what the sentence actually says :
"Están pintando la puerta" : "they are painting the door"
Están pintando la puerta. means: They are painting the door. The pronoun is often left out.
Está pintada la puerta or La puerta está pintada. means: The door is painted.
Please note: if you write or say, the door is painted (either one of the previous sentences), which is in the passive form, the noun has to be in agreement with the adjective: they are both feminine. La puerta has to agree with past participle of pintar (pintada) since the verb estar is used.
In the sense of: it is not bare wood.