Publishing Container to Quay by Example

I'm trying to figure out how to publish a versioned Docker container to Quay.io and am not seeing how to do so in their docs. From the docs:

# Login to Quay.io
docker login quay.io (will now be prompted for username + password)

# Get your <containerID>
docker ps -l

# Tag that container by <containerId>
docker commit <containerId> quay.io/<myUsername>/<myRegistry>

# Now publish it to Quay.io
docker push quay.io/<myUsername>/<myRegistry>

However this leaves me very confused:

What if I want my containers tagged/versioned with, say, 1.0.0-SNAPSHOT, or 1.0-rc, or 2.0 or 3.1.6-G.A? Where do I specify the actual tag name/version?
When I run the docker push command, how does Docker know which (tagged/versioned) container to publish?

Solution

First, it's worth noting that you don't push containers to a registry. You push images. The docker commit command in your question creates new image based on an active container.

When assigning tags to images -- using either the docker tag command, which is more common [1], or when using docker commit -- the format of a tag is [<registry>/]<repository>[:<tag>], where [...] is used to represent an option component. So for example you might tag an image targeting your account on Docker Hub like this:

docker tag smeeb/webserver

This would be assigned the latest tag, since you haven't specified one explicitly. To explicitly assign the tag awesome instead of latest:

docker tag smeeb/webserver:awesome

When you're tagging an image for an alternate registry, you include the registry name as part of the tag, as in:

docker tag <imageId> quay.io/smeeb/webserver

Here, quay.io is the registry, smeeb/webserver is the repository, and you've implicitly assigned the latest tag. You can be explicit instead:

docker tag <imageId> quay.io/smeeb/webserver:1.0.0-SNAPSHOT

And so forth. You can use the same syntax when creating images with docker commit.

[1]: The reason I saw that using docker tag is more common is that use of docker commit is generally considered an anti-pattern. Use a Dockerfile to create your new image, which makes it much easier to reproduce the same configuration (and allows you to version control the configuration of your image).