Identify regions of zeros that are surrounded by ones in a matrix

I have a list of binary matrices. In each matrix, I want to detect regions of white pixels (0) surrounded by ring (a chain) of connected black pixels (1).

For example, in the matrix below, there are two regions of white pixels (zeros) both entirely surrounded by a "chain" of connected 1s: the 2x2 and the 3x2 group of 0s.

m
#         [,1] [,2] [,3] [,4] [,5] [,6] [,7]
#    [1,]    1    1    1    1    0    0    1
# -> [2,]    1    0    0    1    1    1    1
# -> [3,]    1    0    0    1    0    0    1 <- 
#    [4,]    1    1    1    1    0    0    1 <- 
#    [5,]    1    0    0    1    0    0    1 <-
#    [6,]    0    1    1    1    1    1    1

m <- matrix(c(1, 1, 1, 1, 0, 0, 1,
              1, 0, 0, 1, 1, 1, 1,
              1, 0, 0, 1, 0, 0, 1,
              1, 1, 1, 1, 0, 0, 1,
              1, 0, 0, 1, 0, 0, 1,
              0, 1, 1, 1, 1, 1, 1),
            byrow = TRUE, nrow = 6)

An example with three binary matrices in a list:

set.seed(12345)
x <- matrix(sample(c(0,1), 225, prob=c(0.8,0.2), replace=TRUE), nrow = 15)

set.seed(9999)
y <- matrix(sample(c(0,1), 225, prob=c(0.8,0.2), replace=TRUE), nrow = 15)

set.seed(12345)
z <- matrix(sample(c(0,1), 225, prob=c(0.8,0.2), replace=TRUE), nrow = 15)

mat_list <- list(x, y, z)

I have thought of using the boundaries function in the raster package, so I start by converting the matrices to raster:

library(igraph)
library(raster)

lapply(list, function (list) {
  Rastermat <- raster(list)
})

Any guidance on how I could implement this would be appreciated.