R shiny application with non-root output. Go to http://localhost:3838/myapp to see the shiny application.

0-readme.txt

$ tree
├── app
│   └── myapp
│       └── app.R
└── compose.yml
$ mkdir -p app/myapp
$ sudo chmod 777 app/myapp # shiny user can write
$ docker compose up

# Click the 'Write Output File' button
$ ls -l app/myapp
$ cat app/myapp/output.txt
Written at 2025-04-09 02:04:08.826596

app.R

library(shiny)

ui <- fluidPage(
  titlePanel("Write File Example"),
  actionButton("write_btn", "Write Output File"),
  verbatimTextOutput("status"),
  verbatimTextOutput("uid_info")
)

server <- function(input, output, session) {
  output$uid_info <- renderPrint({
    list(
      user = Sys.info()[["user"]],
      uid = system("id -u", intern = TRUE),
      gid = system("id -g", intern = TRUE),
      pwd = getwd()
    )
  })

  observeEvent(input$write_btn, {
    file_path <- "output.txt"
    tryCatch({
      writeLines(paste("Written at", Sys.time()), file_path)
      output$status <- renderText(paste("Successfully wrote to:", file_path))
    }, error = function(e) {
      output$status <- renderText(paste("Error:", e$message))
    })
  })
}

shinyApp(ui, server)

compose.yml

services:
  shiny:
    image: rocker/shiny
    ports:
      - "3838:3838"
    volumes:
      - ./app:/srv/shiny-server
    user: "${UID}:${GID}"