Helm Chart: Pin version of Docker image used

Currently, we set the Docker image tag of `stac-auth-proxy` to be `latest` by default: https://github.com/developmentseed/stac-auth-proxy/blob/main/helm/values.yaml#L8

It seems like it might be nicer to pin the version, am just not sure what the best way to do that might be.

On other projects, we have used [`chartpress`](https://github.com/jupyterhub/chartpress) to build the docker image, tag the image and handle modifying the `values` files with the tag of the just-built image, like https://github.com/developmentseed/osm-seed/blob/develop/.github/workflows/chartpress.yaml

I feel adding chartpress is a bit of unnecessary complexity - it's really useful if one is building multiple images in the same repository, but I think it's a bit much just to handle the image tagging.

I think a question also is: Do we want to automatically publish a new Helm chart version with an updated Image tag every time we publish a new Docker image, OR do we want this to be an explicit / manual action to edit the image version in the helm chart?

### Option 0: Do nothing, and leave the default image tag at `latest`

I do not like this option as this could mean that the image version running on a deployment could randomly change and it becomes hard to reason about.

We **could** make this the user's responsibility to pin in their deployment, but I believe we should have good defaults, and `latest` does not seem like a good default to me.

If we really want to do this, I might remove the default entirely and make image tag a required `value` to pass in, and then it's the users responsibility to handle choosing a stable version of `stac-auth-proxy` and updating when they want to.

### Option 1: Automate updating of image tag in helm chart every time an image is updated

I like this as it keeps our latest helm chart up-to-date with the latest image. It does mean that we will then also update the helm chart for every new docker version (which I think is fine).

So far, we've used the `chartpress` tool to automate this on other projects. If we want this, I can do some looking around to see if we can find something a bit simpler, since `chartpress` also does a lot more that we don't really need.

### Option 2: Explicitly update version in Helm Chart when we want to roll out a release.

This would make updating the Docker image + Helm Chart a two step process: push update to Docker image and build, and then update the default `values.yaml` in the helm chart with the new image tag.

This makes the helm release a bit more intentional and forces us to consider if we need to make changes to the helm chart for new changes in the application, etc.

I don't really like this either - we will forget, things will drift, etc. - I think it's better to have a good test suite for the helm chart, and just automatically update it to the latest docker image version.

### Decision

I would go with Option 1 and just automate the version updating and pinning process. Will be great to hear if anyone has any thoughts on this, or experience with tooling to automate the docker tagging -> populating helm values (or an alternate approach). I imagine this is a fairly common use-case for projects where we are building the docker image and publishing the helm chart out of the same repository. We've definitely wanted this often and so far used `chartpress` - but I'm leaning away from `chartpress` for repositories that build a single image, so would look for an elegant way to do this.

cc @emmanuelmathot @sunu @ividito @pantierra @alukach