Docker build and docker-compose build with user namespace mapping - docker

Question

I have enabled user namespace mapping in docker and building an image using docker build works but when I use docker-compose for the image it fails with below message. What can be the reason for it?
db#vagrant:~/docker$ docker-compose up --build
Building db
Step 1/3 : FROM alpine:latest
---> e7d92cdc71fe
Step 2/3 : WORKDIR /app
---> Using cache
---> 1491149423a1
Step 3/3 : COPY 1.txt .
ERROR: Service 'db' failed to build: failed to copy files: failed to copy file: Container ID 65536 cannot be mapped to a host ID
My user id is generated by some setup scripts which results in UID with larger than 65535 value.
db#vagrant:~/docker$ id
uid=65536(db) gid=1000(db) groups=1000(db),27(sudo),998(docker)
Docker configuration for namespace mapping
db#vagrant:~/docker$ cat /etc/docker/daemon.json
{
"userns-remap": "db"
}
db#vagrant:~/docker$ cat /etc/subuid /etc/subgid
db:100000:65536
db:100000:65536
Dockerfile contents(1.txt is empty file)
db#vagrant:~/docker$ cat Dockerfile
FROM alpine:latest
WORKDIR /app
COPY 1.txt .
docker-compose.yml file contents
db#vagrant:~/docker$ cat docker-compose.yml
version: "2"
services:
db:
build:
context: .
dockerfile: Dockerfile
image: sirishkumar/test
Output of docker build command
db#vagrant:~/docker$ docker build -t sirishkumar/test .
Sending build context to Docker daemon 3.584kB
Step 1/3 : FROM alpine:latest
latest: Pulling from library/alpine
c9b1b535fdd9: Pull complete
Digest: sha256:ab00606a42621fb68f2ed6ad3c88be54397f981a7b70a79db3d1172b11c4367d
Status: Downloaded newer image for alpine:latest
---> e7d92cdc71fe
Step 2/3 : WORKDIR /app
---> Running in 55f092b96268
Removing intermediate container 55f092b96268
---> 8af079e6a478
Step 3/3 : COPY 1.txt .
---> b3c14a691102
Successfully built b3c14a691102
Successfully tagged sirishkumar/test:latest
Output of docker-compose
db#vagrant:~/docker$ docker-compose up --build
Creating network "docker_default" with the default driver
Building db
Step 1/3 : FROM alpine:latest
latest: Pulling from library/alpine
c9b1b535fdd9: Pull complete
Digest: sha256:ab00606a42621fb68f2ed6ad3c88be54397f981a7b70a79db3d1172b11c4367d
Status: Downloaded newer image for alpine:latest
---> e7d92cdc71fe
Step 2/3 : WORKDIR /app
---> Running in fe39955aed1a
Removing intermediate container fe39955aed1a
---> fb23b8888f4a
Step 3/3 : COPY 1.txt .
ERROR: Service 'db' failed to build: failed to copy files: failed to copy file: Container ID 65536 cannot be mapped to a host ID

Answer 1

You have a range of 65,536 user id's to map into your docker user namespace:
db#vagrant:~/docker$ cat /etc/subuid /etc/subgid
db:100000:65536
db:100000:65536
And then you're telling docker to copy a file into the container owned by an ID outside of that range (user ID's start at 0):
db#vagrant:~/docker$ id
uid=65536(db) gid=1000(db) groups=1000(db),27(sudo),998(docker)
You need to set your user id on the host to be within the host user id range (less than 65536).