The colloquial British-English phrase early doors means early on, at an early stage.

It frequently occurs in the context of football (i.e., the game played under the rules of the Football Association).

The origin of this phrase is unknown—as explained (despite several inaccuracies) in the column Bush telegraph, published in The Daily Telegraph (London, England) of Tuesday 23^rd January 1996:

A PHRASE which crops up often in television and radio sport commentary is “early doors”, as in: “They’ve come out here firing, have Newcastle, Des, and they’ve run United ragged early doors.”
Never used outside the football context, and of hotly disputed derivation, the only certainty about “early doors” is that it translates in English to “early”, or “early on”.
The first recorded use is credited to the current Coventry City manager Ron Atkinson (Big Ron Atkinson, to style him correctly) in the early Eighties, but its usage has recently become pandemic within the sport.
As in its derivation, there are two leading schools of thought. One camp, the “bastardisers”, postulates that the phrase is simply a corruption of “early days”, the confusion stemming from variance in regional accent; they adduce the lack of any corollary (there is no such thing as “late doors”) in support of this.
The rival school, the so called “temporalists”, posits that the origin lies in the Thirties golden age of cinema, “early doors” being northern vernacular for a matinee, when tickets were at their cheapest *; a rogue grouping from within this school prefers the notion of a pub opening its doors for business.
In truth, little of this is verifiable and, although the debate will go on, “early doors” seems certain to be one of the more enduring etymological enigmas of the age.

[* In fact, originally in the late 19^th century, the phrase early door(s) denoted early admission to a theatrical performance, typically for a higher price.]

The earliest occurrences of the phrase early doors—meaning early on, at an early stage—that I have found are as follows, in chronological order:

1-: From Allestree sign on for new season, by Alan Smith, published in the Derby Evening Telegraph (Derby, Derbyshire, England) of Saturday 24^th July 1976:

The committee of Allestree Jazz Club are […] marshallng [sic] their programme for their seventh season on the Derby area jazz scene.
[…]
But to put on star jazz in the bundles presented by Allestree JC, and at a reasonable admission price depends on a loyal corps of members who can be relied upon to support the monthly concerts almost 100 per cent.
So to get things off to a promising start the first show on Wednesday, September 1, will be at a specially reduced charge to members who have already pledged their support by joining in advance, or who join on the night.
[…]
If you want to get in early doors on the membership register, pop a pound into an envelope (with a stamp and addressed envelope for reply) to Mrs Sylvia Limbert, 28 Woodford Road, Mackworth Estate, Derby; or give her a ring—Derby 40509.
You will then be set to savour at preferential rates: The Monty Sunshine Band (September 16), The Chris Barber Band (October 20), Terry Lightfoot Jazz Band (November 3) and the Kelly Ball Jazzmen (December 15).

2-: From the column Talking Shop, by Janet Buckton, published in the Coventry Evening Telegraph (Coventry, Warwickshire, England) of Tuesday 22^nd August 1978:

REMEMBER the days when, spring weather or not, you had to have a new pair of shoes because the season had arrived.
The tale was two or four pairs a year, bought rigidly to the seasonal calendar. Smart shoes would last a season; casual shoes, which we bought fewer of, would last a year or two.
But we’ve changed.
A clued-up retailer would be mad if he didn’t listen to the weather forecast all day.
“If it sets in to rain,” one wholesaler told me, “we’ll have retailers arriving early doors to buy up more shiny wellies. And if he’s got any sense, he’ll have them displayed outside the shop.”

3-: From Motivation!, an interview of the British football manager Brian Clough (1935-2004) by Hugh McIlvanney, published in The Observer (London, England) of Sunday 18^th November 1979:

‘My attitude to players may have changed a bit. I’ll always demand their respect just as I am prepared to respect them. Early doors it was vital to me that they liked me, too. But I became so attached to them as players that when I left Derby I found I liked them more than they liked me. So the liking business recedes with every year of cynical life that wraps itself around me inside and outside of football.’

4-: From the account of a soccer match between Aston Villa and West Ham, by Ray Matts, published in the Sports Argus (Birmingham, West Midlands, England) of Saturday 17^th October 1981:

There was a certain sloppiness about Villa’s passing in the early stages—especially the supply from the back four to midfield—and sadly, too, little evidence of Vill’s [sic] potential to recover from their early setback.
However it was early doors.
Villa’s most promising move so far came when Williams threaded the ball forward down the right wing to Geddis who fed Bremner. The midfield man crossed for Shaw, who was off balance to launch a dipping header that flew only just over the crossbar.
Villa Park erupted into cheers of relief in the 22^nd minute when TONY MORLEY equalised for the home club.

5-: From the Evening Post (Nottingham, Nottinghamshire, England) of Friday 15^th June 1984:

Ruthyn defies grader

RUTHYN STAR, owned by Ken Hirst and trained by Arthur Walters, defied the grader by winning her third race on the trot last night at Colwick Park Greyhound Stadium, writes JACK BEAN.
Although she tends to win her races by narrow margins, she certainly sticks her neck out near the “jam stick” and once again Ruthyns [sic] Star was the subject of a successful gamble.
Laid early doors at 3-1, on traps rise it was ‘twos the field’ and she left the layers licking their wounds when catapulting home in a calculated 34.26 (530m).