It sounds like, in the movie, the doctor is shown to have a generic kit for the scene. I'm not sure how historically accurate the film was trying to be, either. So I'll answer this as though the doctor simply has a basic kit, though each doctor likely carried a kit that was more in step with his daily duties. E.g. A doctor that served mostly elderly clientele wouldn't carry obstetrics tools as a matter of course.

So, timeframe, the Spanish Civil war took place in the mid 1930s, before penicillin (usually understood as the first real antibiotic) was being used but about half of the war too place after the invention of Sulfa drugs, a precursor to modern antibiotics. So Sulfa might have been the antibiotic mentioned in the film. (I've seen the film, and I know that the subtitles are a little loose -- it would be helpful to know what word they actually used.) Sulpha, like aspirin, went through a period of being handed out like candy after it was synthesized, which makes me more inclined to believe that the film was intending to portray sulfa.

The liquid sedative is almost certainly intended to be morphine, though it's possible that the drug been shown during the birth scene might have been intended to be scopolamine, which was used to put mothers in labor into a 'twilight' sleep when combined with other drugs. Heroin had been banned in most nations by 1928, but morphine and scopolamine were still widespread (and recently made better via improved farming and preparation techniques.)

I think that the drug used to euthanize the soldier was also likely morphine. It was still considered a miracle drug by many people in that time period and large doses behave a lot like phenobarbital/sodium thiopental (the set of shots used to euthanize animals) -- the patient goes to sleep and then passes away when the heart is depressed. However, these drugs wouldn't have been typical in a doctors BAG during any time period (phenobarbital would have been likely available at hospitals, sanitariums, etc.)

Doctors that made house calls during that time tended to vary both in quality and theory -- this is why things like the Joslin Institute saw people from all over the world. But for the sake of sanity, I'm going to assume that he was a typical doctor of his time, and he had access to a reliable source of supplies despite the war. His bag would have contained the following drugs: morphine, sulfa, and aspirin. Those appear to also be the drugs that the film intended to portray. His bag also might have contained his chemist's particular version of cough suppressant, mineral spirits, castor oil (still considered a solution for malingering and constipation....) and his chemist's particular version of multi-purpose 'salve'. Chemist, here, indicates a chemist specifically making drugs. Pharamcists.chemists, at the time, received information from drug companies on how to synthesize the drug and made their own batches of various medicines. They also had their own medicines for common ailments.

While I know they are specific to diabetes, these two books provide very compelling looks into how medicine worked in the 1930s: the 1968 "Diabetic Manual for Doctor and Patient" by Joslin himself and Bittersweet : diabetes, insulin, and the transformation of illness by Chris Feudtner.