Use a pinned devcontainer base image version (#50)

We want to pin to the most specific version we can, to ensure that the
devcontainer builds are always reproducible.

This is despite the general guidance from Microsoft for these images
being to use ["the major release version of this tag to avoid breaking
changes while still taking fixes and content additions as they land.
e.g. `0-buster`"][1]

We will still take fixes and content additions as they land, as we use
Dependabot to keep us up to date.  Dependabot will create a PR whenever
there is a new version of the base image, and we then manually apply
that new version to the `devcontainer.json` (full details on how to do
this are in the `DEPENDENCIES.md`).

See also [this documentation on the base image versions][2]

[1]: https://hub.docker.com/_/microsoft-vscode-devcontainers?tab=description
[2]: https://github.com/microsoft/vscode-dev-containers/tree/master/containers/debian#using-this-definition-with-an-existing-folder

.devcontainer/Dockerfile

@@ -1,7 +1,7 @@
 # Based on the example Dockerfile at:
 # https://github.com/microsoft/vscode-dev-containers/blob/master/containers/debian/.devcontainer/Dockerfile
 
-# Update the VARIANT arg in devcontainer.json to pick an Debian version: buster (or debian-10), stretch (or debian-9)
+# Update the VARIANT arg in devcontainer.json to override the Debian version below
 ARG VARIANT=buster
 FROM mcr.microsoft.com/vscode/devcontainers/base:${VARIANT}
 

.devcontainer/devcontainer.json

@@ -1,6 +1,14 @@
 {
 	"name": "Docker Container GitHub Action development",
-	"dockerFile": "Dockerfile",
+	"build": {
+		"dockerfile": "Dockerfile",
+		// Update 'VARIANT' to pick an Debian version: buster, stretch, or a version
+		// specific tag tied to a release in:
+		// https://github.com/microsoft/vscode-dev-containers/releases
+		// See also: 
+		// https://github.com/microsoft/vscode-dev-containers/tree/master/containers/debian#using-this-definition-with-an-existing-folder
+		"args": { "VARIANT": "0.123.0-buster" }
+	},
 
 	// Set *default* container specific settings.json values on container create.
 	"settings": { 

.github/DEPENDENCIES.md

@@ -29,6 +29,22 @@ but that would be overkill for now.
 Eventually as Dependabot adds more features we may be able to remove this workaround.
 
 
+## Devcontainer base image version
+
+We always pin to the most specific ([patch-level](https://semver.org/)) version of the [devcontainer Docker base image](https://github.com/microsoft/vscode-dev-containers/tree/master/containers/debian#using-this-definition-with-an-existing-folder) that we can, to ensure that the devcontainer builds are always reproducible.  
+
+This is despite the general guidance from Microsoft for these images being to use ["the major release version of this tag to avoid breaking changes while still taking fixes and content additions as they land. e.g. `0-buster`"](https://hub.docker.com/_/microsoft-vscode-devcontainers?tab=description)
+
+The reason we are comfortable pinning to a specific patch-level version is that we still take fixes and content additions as they land, because we use Dependabot to keep us up to date (using the [above workaround](#workaround-for-other-dependencies)).
+
+NB when we manually update the version in the `devcontainer.json`, we need to be careful as the version format is
+e.g. `0.123.0-buster`, compared to `v0.123.0` in the [`dependabot_hack.yml`](workflows/dependabot_hack.yml)
+
+When the new version is a patch or minor level update, we don't bother to test manually that the new remote container works (before merging the Dependabot PR).  In the (hopefully rare) event that it doesn't, we can simply put in another PR to downgrade the version to one that does work.  
+
+For major version updates, we should test that the remote container works successfully before merging the Dependabot PR.  The most failsafe way to test this is to [open the Dependabot PR branch in a remote Visual Studio Code container](https://code.visualstudio.com/docs/remote/containers#_quick-start-open-a-git-repository-or-github-pr-in-an-isolated-container-volume).  Alternatively starting a new [Codespace](https://github.com/features/codespaces/) that points to the Dependabot PR branch should also be a valid test.
+
+
 ## Dockerfile dependencies
 
 We have [pinned the linux dependencies in the devcontainer Dockerfile](https://github.com/agilepathway/hoverfly-github-action/pull/46/files), but there is no mechanism to automatically update them, currently.  It looks like [it's on Dependabot's roadmap](https://github.com/dependabot/dependabot-core/issues/2129#issuecomment-511552345), so we have reminders every 6 months to 

.github/workflows/dependabot_hack.yml

@@ -28,3 +28,7 @@ jobs:
 
       # update the version in `github_tag_and_release.yml` manually, too
       - uses: golang/[email protected]
+
+      # update the variant version in the devcontainer.json manually, too
+      # (see ../DEPENDENCIES.md#devcontainer-base-image-version for more info )
+      - uses: microsoft/[email protected]