### k8s.gcr.io/etcd docker image

Provides docker images containing etcd and etcdctl binaries for multiple etcd
version as well as a migration operator utility for upgrading and downgrading
etcd--it's data directory in particular--to a target version.

#### Versioning

Each `k8s.gcr.io/etcd` docker image is tagged with an version string of the form
`<etcd-version>-<image-revision>`, e.g. `3.0.17-0`.  The etcd version is the
SemVer of latest etcd version available in the image. The image revision
distinguishes between docker images with the same lastest etcd version but
changes (bug fixes and backward compatible improvements) to the migration
utility bundled with the image.

In addition to the latest etcd version, each `k8s.gcr.io/etcd` image contains
etcd and etcdctl binaries for older versions of etcd. These are used by the
migration operator utility when performing downgrades and multi-step upgrades,
but can also be used as the etcd target version.

#### Usage

Always run `/usr/local/bin/migrate` (or the
`/usr/local/bin/migrate-if-needed.sh` wrapper script) before starting the etcd
server.

`migrate` writes a `version.txt` file to track the "current" version
of etcd that was used to persist data to disk. A "target" version may also be provided
by the `TARGET_STORAGE` (e.g. "etcd3") and `TARGET_VERSION` (e.g. "3.4.13" )
environment variables. If the persisted version differs from the target version,
`migrate-if-needed.sh` will migrate the data from the current to the target
version.

Upgrades to any target version are supported. The data will be automatically upgraded
in steps to each minor version until the target version is reached.

Downgrades to the previous minor version of the 3.x series is supported.

#### Permissions

By default, `migrate` will write data directory files with default permissions
according to the umask it is run with. When run in the published
`k8s.gcr.io/etcd` images the default umask is 0022 which will result in 0755
directory permissions and 0644 file permissions.

#### Cross building

For `amd64`, official `etcd` and `etcdctl` binaries are downloaded from Github
to maintain official support.  For other architectures, `etcd` is cross-compiled
from source. Arch-specific `busybox` images serve as base images.

#### How to release

First, update `ETCD_VERSION` and `REVSION` in the `Makefile`.

Next, build and test the image:

```console
$ make build test
```

Last, build and push the docker images for all supported architectures.

```console
# Build images for all the architecture and push the manifest image as well
$ make all-push

# Build images for all the architecture
$ make all-build

# Build image for target architecture(default=amd64)
$ make build ARCH=ppc64le
```

If you don't want to push the images, run `make` or `make build` instead
