docs: improve dev containers documentation for user start-up (#15458)

- [x] TODO: verify and expand [What is an image, template, devcontainer,
or
workspace](https://coder.com/docs/@bp-dev-containers/admin#what-is-an-image-template-devcontainer-or-workspace)
- [x] TODO: verify and expand `## Add a devcontainer template to Coder`
- [x] TODO: verify and expand `## Layer and image caching`
- [x] TODO: link to and edit
[dotfiles](https://coder.com/docs/user-guides/workspace-dotfiles) doc
- [x] TODO:
https://github.com/coder/coder/pull/15458/files#diff-be0dd9ff983020129c7a94c628a0fc4a397a9bbc8b5eec92ec38f4b8c83fe167R10


[preview](https://coder.com/docs/@bp-dev-containers/admin/templates/managing-templates/devcontainers)

---------

Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>
Co-authored-by: Cian Johnston <cian@coder.com>
Co-authored-by: Ben Potter <ben@coder.com>
Co-authored-by: Edward Angert <2408959-EdwardAngert@users.noreply.gitlab.com>
Co-authored-by: Muhammad Atif Ali <atif@coder.com>

docs/admin/index.md

@@ -15,4 +15,56 @@ and [API](../reference/api/index.md) docs.
 For any information not strictly contained in these sections, check out our
 [Tutorials](../tutorials/index.md) and [FAQs](../tutorials/faqs.md).
 
+## What is an image, template, dev container, or workspace
+
+**Image**
+
+- A [base image](./templates/managing-templates/image-management.md) contains
+  OS-level packages and utilities that the Coder workspace is built on. It can
+  be an [example image](https://github.com/coder/images), custom image in your
+  registry, or one from [Docker Hub](https://hub.docker.com/search). It is
+  defined in each template.
+- Managed by: Externally to Coder.
+
+**Template**
+
+- [Templates](./templates/index.md) include infrastructure-level dependencies
+  for the workspace. For example, a template can include Kubernetes
+  PersistentVolumeClaims, Docker containers, or EC2 VMs.
+- Managed by: Template administrators from within the Coder deployment.
+
+**Startup scripts**
+
+- Agent startup scripts apply to all users of a template. This is an
+  intentionally flexible area that template authors have at their disposal to
+  manage the "last mile" of workspace creation.
+- Managed by: Coder template administrators.
+
+**Workspace**
+
+- A [workspace](../user-guides/workspace-management.md) is the environment that
+  a developer works in. Developers on a team each work from their own workspace
+  and can use [multiple IDEs](../user-guides/workspace-access/index.md).
+- Managed by: Developers
+
+**Development containers (dev containers)**
+
+- A
+  [Development Container](./templates/managing-templates/devcontainers/index.md)
+  is an open-source specification for defining development environments (called
+  dev containers). It is generally stored in VCS alongside associated source
+  code. It can reference an existing base image, or a custom Dockerfile that
+  will be built on-demand.
+- Managed by: Dev Teams
+
+**Dotfiles / personalization**
+
+- Users may have their own specific preferences relating to shell prompt, custom
+  keybindings, color schemes, and more. Users can leverage Coder's
+  [dotfiles support](../user-guides/workspace-dotfiles.md) or create their own
+  script to personalize their workspace. Be aware that users with root
+  permissions in their workspace can override almost all of the previous
+  configuration.
+- Managed by: Individual Users
+
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