It is appropriate to use COPY in this case.

examples/cc/Dockerfile

@@ -1,5 +1,5 @@
 FROM gcc:6 AS build-env
-ADD . /app
+COPY . /app
 WORKDIR /app
 RUN cc hello.c -o hello
 

examples/dotnet/Dockerfile

@@ -1,5 +1,5 @@
 FROM microsoft/dotnet:2.0.0-sdk AS build-env
-ADD . /app
+COPY . /app
 WORKDIR /app
 RUN dotnet restore -r linux-x64
 RUN dotnet publish  -c Release -r linux-x64

examples/go/Dockerfile

@@ -1,7 +1,7 @@
 FROM golang:1.12 as build-env
 
 WORKDIR /go/src/app
-ADD . /go/src/app
+COPY . /go/src/app
 
 RUN go get -d -v ./...
 

examples/java/Dockerfile

@@ -1,5 +1,5 @@
 FROM openjdk:11-jdk-slim AS build-env
-ADD . /app/examples
+COPY . /app/examples
 WORKDIR /app
 RUN javac examples/*.java
 RUN jar cfe main.jar examples.HelloJava examples/*.class 

examples/nodejs/Dockerfile

@@ -1,5 +1,5 @@
 FROM node:10 AS build-env
-ADD . /app
+COPY . /app
 WORKDIR /app
 
 RUN npm ci --only=production

examples/python2.7/Dockerfile

@@ -1,5 +1,5 @@
 FROM python:2.7-slim AS build-env
-ADD . /app
+COPY . /app
 WORKDIR /app
 
 FROM gcr.io/distroless/python2.7

examples/python3/Dockerfile

@@ -1,5 +1,5 @@
 FROM python:3-slim AS build-env
-ADD . /app
+COPY . /app
 WORKDIR /app
 
 FROM gcr.io/distroless/python3

examples/rust/Dockerfile

@@ -1,6 +1,6 @@
 FROM rust:1.41.0 as build-env
 WORKDIR /app
-ADD . /app
+COPY . /app
 RUN cargo build --release
 
 FROM gcr.io/distroless/cc