Docker build cache does not ignore uid/gid on ADD or COPY #32816
Description
The uid/gid of a file (on the build host's filesystem) seem to have an effect on the hash of a file when it is added through ADD or COPY. This seems superfluous since the ownership of the added file is reset to uid=gid=0 in the Docker image.
This behaviour may lead to the Docker build cache being busted in continuous integration environments, if uid/gid are different across different build nodes.
For a minimum working example, please consider the following example where an image is built twice, the second time with different ownership of a COPY'd file.
Please note
- The hashes of the added file are different as per
docker history - The ownership is reset to uid=gid=0 inside the image, as stated in the documentation for ADD ("All new files and directories are created with a UID and GID of 0.")
$ ls -lah
total 8,0K
drwxrwxr-x 1 jprobst jprobst 36 Apr 25 18:08 .
drwxrwxrwx 1 jprobst jprobst 6,4K Apr 25 17:15 ..
-rw-rw-r-- 1 jprobst jprobst 44 Apr 25 18:08 Dockerfile
-rwxrwxrwx 1 jprobst jprobst 5 Apr 25 17:15 test.txt
$ cat Dockerfile
FROM ubuntu:16.04
COPY test.txt /test.txt
$ docker build -t demo .
Sending build context to Docker daemon 3.072kB
Step 1/2 : FROM ubuntu:16.04
---> 104bec311bcd
Step 2/2 : COPY test.txt /test.txt
---> ea497fe942a0
Removing intermediate container 16753a7ab82b
Successfully built ea497fe942a0
$ docker history demo
IMAGE CREATED CREATED BY SIZE COMMENT
ea497fe942a0 17 seconds ago /bin/sh -c #(nop) COPY file:db60da90f31b3b... 5B
104bec311bcd 4 months ago /bin/sh -c #(nop) CMD ["/bin/bash"] 0B
<missing> 4 months ago /bin/sh -c mkdir -p /run/systemd && echo '... 7B
<missing> 4 months ago /bin/sh -c sed -i 's/^#\s*\(deb.*universe\... 1.9kB
<missing> 4 months ago /bin/sh -c rm -rf /var/lib/apt/lists/* 0B
<missing> 4 months ago /bin/sh -c set -xe && echo '#!/bin/sh' >... 745B
<missing> 4 months ago /bin/sh -c #(nop) ADD file:7529d28035b43a2... 129MB
$ sudo chown 1001:1001 test.txt
$ ls -lah
total 8,0K
drwxrwxr-x 1 jprobst jprobst 36 Apr 25 18:08 .
drwxrwxrwx 1 jprobst jprobst 6,4K Apr 25 17:15 ..
-rw-rw-r-- 1 jprobst jprobst 44 Apr 25 18:08 Dockerfile
-rwxrwxrwx 1 1001 1001 5 Apr 25 17:15 test.txt
$ docker build -t demo .
Sending build context to Docker daemon 3.072kB
Step 1/2 : FROM ubuntu:16.04
---> 104bec311bcd
Step 2/2 : COPY test.txt /test.txt
---> 43cb126c18bb
Removing intermediate container 4e729efe09bb
Successfully built 43cb126c18bb
$ docker history demo
IMAGE CREATED CREATED BY SIZE COMMENT
43cb126c18bb 14 seconds ago /bin/sh -c #(nop) COPY file:f9b82392428347... 5B
104bec311bcd 4 months ago /bin/sh -c #(nop) CMD ["/bin/bash"] 0B
<missing> 4 months ago /bin/sh -c mkdir -p /run/systemd && echo '... 7B
<missing> 4 months ago /bin/sh -c sed -i 's/^#\s*\(deb.*universe\... 1.9kB
<missing> 4 months ago /bin/sh -c rm -rf /var/lib/apt/lists/* 0B
<missing> 4 months ago /bin/sh -c set -xe && echo '#!/bin/sh' >... 745B
<missing> 4 months ago /bin/sh -c #(nop) ADD file:7529d28035b43a2... 129MB
$ docker run -t demo ls -lah /test.txt
-rwxrwxrwx 1 root root 5 Apr 25 15:15 /test.txt