print column name when specific row of a column contains string in R

I have the following dataset

A    B    C    D1!   0!   1!   0!0!   .    1!   1!

I need to print the column name to each match as follows

matches_1     matches_0A,C           B,DC,D           A

I'm using:

df$matches_1 <- colSums(apply(df, 1, stringr::str_count, "1!"))

to count occurences of 1! in each row, but i'm not sure how to print the column name instead of the sum