build¶

Build an image from a Dockerfile and put it in the storage directory. Use ch-run(1) to execute RUN instructions.

Required argument:

Options:

ch-image is a fully unprivileged image builder. It does not use any setuid or setcap helper programs, and it does not use configuration files /etc/subuid or /etc/subgid. This contrasts with the “rootless” or “fakeroot” modes of some competing builders, which do require privileged supporting code or utilities.

This approach does yield some quirks. We provide built-in workarounds that should mostly work (i.e., --force), but it can be helpful to understand what is going on.

ch-image executes all instructions as the normal user who invokes it. For RUN, this is accomplished with ch-run -w --uid=0 --gid=0 (and some other arguments), i.e., your host EUID and EGID both mapped to zero inside the container, and only one UID (zero) and GID (zero) are available inside the container. Under this arrangement, processes running in the container for each RUN appear to be running as root, but many privileged system calls will fail without the workarounds described below. This affects any fully unprivileged container build, not just Charliecloud.

The most common time to see this is installing packages. For example, here is RPM failing to chown(2) a file, which makes the package update fail:

  Updating   : 1:dbus-1.10.24-13.el7_6.x86_64                            2/4
Error unpacking rpm package 1:dbus-1.10.24-13.el7_6.x86_64
error: unpacking of archive failed on file /usr/libexec/dbus-1/dbus-daemon-launch-helper;5cffd726: cpio: chown


  Cleanup    : 1:dbus-libs-1.10.24-12.el7.x86_64                         3/4
error: dbus-1:1.10.24-13.el7_6.x86_64: install failed

This one is (ironically) apt-get failing to drop privileges:

E: setgroups 65534 failed - setgroups (1: Operation not permitted)
E: setegid 65534 failed - setegid (22: Invalid argument)
E: seteuid 100 failed - seteuid (22: Invalid argument)
E: setgroups 0 failed - setgroups (1: Operation not permitted)

By default, nothing is done to avoid these problems, though ch-image does try to detect if the workarounds could help. --force activates the workarounds: ch-image injects extra commands to intercept these system calls and fake a successful result, using fakeroot(1). There are three basic steps:

The details are specific to each distribution. ch-image analyzes image content (e.g., grepping /etc/debian_version) to select a configuration; see lib/fakeroot.py for details. ch-image prints exactly what it is doing.

storage-path¶

Print the storage directory path and exit.

pull¶

Pull the image described by the image reference IMAGE_REF from a repository by HTTPS. See the FAQ for the gory details on specifying image references.

This script does a fair amount of validation and fixing of the layer tarballs before flattening in order to support unprivileged use despite image problems we frequently see in the wild. For example, device files are ignored, and file and directory permissions are increased to a minimum of rwx------ and rw------- respectively. Note, however, that symlinks pointing outside the image are permitted, because they are not resolved until runtime within a container.

Destination argument:

Options:

Context directory¶

The context directory is bind-mounted into the build, rather than copied like Docker. Thus, the size of the context is immaterial, and the build reads directly from storage like any other local process would. However, you still can’t access anything outside the context directory.

Authentication¶

ch-image can authenticate using one-time passwords, e.g. those provided by a security token. Unlike docker login, it does not assume passwords are persistent.

Environment variables¶

Variable substitution happens for all instructions, not just the ones listed in the Dockerfile reference.

ARG and ENV cause cache misses upon definition, in contrast with Docker where these variables miss upon use, except for certain cache-excluded variables that never cause misses, listed below.

Like Docker, ch-image pre-defines the following proxy variables, which do not require an ARG instruction. However, they are available if the same-named environment variable is defined; --build-arg is not required. Changes to these variables do not cause a cache miss.

HTTP_PROXY
http_proxy
HTTPS_PROXY
https_proxy
FTP_PROXY
ftp_proxy
NO_PROXY
no_proxy

The following variables are also pre-defined:

PATH=/ch/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
TAR_OPTIONS=--no-same-owner

Note that ARG and ENV have different syntax despite very similar semantics.

COPY¶

Especially for people used to UNIX cp(1), the semantics of the Dockerfile COPY instruction can be confusing.

Most notably, when a source of the copy is a directory, the contents of that directory, not the directory itself, are copied. This is documented, but it’s a real gotcha because that’s not what cp(1) does, and it means that many things you can do in one cp(1) command require multiple COPY instructions.

Also, the reference documentation is incomplete. In our experience, Docker also behaves as follows; ch-image does the same in an attempt to be bug-compatible.

RUN mkdir -p /foo/bar && touch /foo/bar/baz
COPY foo /foo

We expect the following differences to be permanent:

Features we do not plan to support¶

build¶

Build image bar using ./foo/bar/Dockerfile and context directory ./foo/bar:

$ ch-image build -t bar -f ./foo/bar/Dockerfile ./foo/bar
[...]
grown in 4 instructions: bar

Same, but infer the image name and Dockerfile from the context directory path:

$ ch-image build ./foo/bar
[...]
grown in 4 instructions: bar

Build using humongous vendor compilers you want to bind-mount instead of installing into a layer:

$ ch-image build --bind /opt/bigvendor:/opt .
$ cat Dockerfile
FROM centos:7
RUN /opt/bin/cc hello.c
#COPY /opt/lib/*.so /usr/local/lib   # fail: COPY doesn't bind mount
RUN cp /opt/lib/*.so /usr/local/lib  # possible workaround
RUN ldconfig

pull¶

Download the Debian Buster image and place it in the storage directory:

$ ch-image pull debian:buster
pulling image:   debian:buster
manifest: downloading
layer 1/1: d6ff36c: downloading
layer 1/1: d6ff36c: listing
validating tarball members
resolving whiteouts
flattening image
layer 1/1: d6ff36c: extracting
done

Same, except place the image in /tmp/buster:

$ ch-image pull debian:buster /tmp/buster
[...]
$ ls /tmp/buster
bin   dev  home  lib64  mnt  proc  run   srv  tmp  var
boot  etc  lib   media  opt  root  sbin  sys  usr