Creating random ratios that add up to 1 by group

I have a dataset as follows:

panelID= c(1:50)
year= c(2005, 2010)
country = c("A", "B", "C", "D", "E", "F", "G", "H", "I", "J")
urban = c("A", "B", "C")
indust = c("D", "E", "F")
sizes = c(1,2,3,4,5)
n <- 2
library(AER)
library(data.table)
library(dplyr)
set.seed(123)
DT <- data.table(   country = rep(sample(country, length(panelID), replace = T), each = n),
                    year = c(replicate(length(panelID), sample(year, n))),
                    sales= round(rnorm(10,10,10),2),
                    industry = rep(sample(indust, length(panelID), replace = T), each = n),
                    urbanisation = rep(sample(urban, length(panelID), replace = T), each = n),
                    size = rep(sample(sizes, length(panelID), replace = T), each = n))
DT <- DT %>%
group_by(country) %>%
mutate(base_rate = as.integer(runif(1, 12.5, 37.5))) %>%
group_by(country, year) %>%
mutate(taxrate = base_rate + as.integer(runif(1,-2.5,+2.5)))
DT <- DT %>%
group_by(country, year) %>%
mutate(vote = sample(c(0,1),1), 
votewon = ifelse(vote==1, sample(c(0,1),1),0))

I would like to add a variable to this dataset called ratio. I want ratio to be a random number between 0 and 1, and I want the sum of these ratios by country to be 1.

How would I go about creating such a column? The only thing I could think of is manually creating vectors which add up to one and then sampling from those vectors.

ratio_sample_6 <- c(0.1, 0.2, 0.3, 0.05, 0.15, 0.2)
DT[,ratio:=sample(ratio_sample_6, replace = FALSE), by="country"]

But even that I could not get to work. Any suggestions?