Added docs for image repository

Author: jhrv

docs/workloads/explanations/imagerepository.md

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+# Image repository
+
+Every team gets it's own image repository where they push the images they run on Nais.
+
+When using the [nais/docker-build-push](https://github.com/nais/docker-build-push)-action in your workflow, this repository is used automatically.
+ 
+## Image Restriction
+
+Nais restricts the use of images from other image registries for two main reasons. 
+
+1. We verify that the image has been uploaded either by a authorized Nais user or Github repository. 
+2. The provided repository uses [Image Streaming](https://cloud.google.com/kubernetes-engine/docs/how-to/image-streaming), ensuring consistently fast start-up times when auto-scaling or during platform maintenance.
+
+## Using third-party images
+
+To use a third-party Docker image, you must upload it to your team’s repository. See the [Upload Third-Party Image](../how-to/upload-image.md) guide for instructions.

docs/workloads/how-to/upload-image.md

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+# Upload third-party image to image repository
+
+This how-to guide will show you how to upload a Docker image to your team's image repository.
+
+## Prerequisites
+
+- [Nais CLI](../../operate/cli/how-to/install.md) installed.
+- A Docker runtime installed and running on your machine
+
+## Log into Nais
+
+```bash
+nais login
+```
+## Ensure you have the image locally
+
+Ensure the desired image is available on your machine by pulling it from its source or building it locally. For example:
+
+```bash
+docker pull repository/my-image:tag --platform linux/amd64
+```
+
+!!! note "Specify the platform"
+    Note that we are adding the `--platform linux/amd64` flag to ensure that the image we are pulling is compatible with the Nais platform.
+    By default the architecture of the image will be the same as the host machine, which might not work.
+
+## Locate the image repository URL
+
+Find the image repository URL on the settings page in the Nais console
+
+`https://console.<<tenant()>>.cloud.nais.io/team/<MY-TEAM>/settings`
+
+The URL will be on this format:
+
+`europe-north1-docker.pkg.dev/nais-management-<ID>/<MY-TEAM>`
+
+## Tag the image
+
+```bash
+docker tag repository/my-image:tag europe-north1-docker.pkg.dev/nais-management-<ID>/<MY-TEAM>/my-image:tag
+```
+
+## Push the image
+
+```bash
+docker push europe-north1-docker.pkg.dev/nais-management-<ID>/<MY-TEAM>/my-image:tag
+```
+