Dev container is slow to load: post-create.sh takes several minutes to finish

[arnaudlh]

Problem

The dev container is slow to load. After creation, the postCreateCommand (bash .devcontainer/post-create.sh) takes a few minutes to finish, blocking productive use of the workspace.

Repro

1. Open the repo in a fresh dev container (Codespaces or VS Code Dev Containers).
2. Wait through container build + features install.
3. Observe post-create.sh running for several minutes before the environment is usable.

Root cause analysis

.devcontainer/post-create.sh runs three relatively heavy installs sequentially on every container creation:

1. Checkov — pip install --user --only-binary :all: checkov — large package with many transitive deps.
2. PSRule.Rules.Azure — Install-Module ... -Scope CurrentUser -Force — pulls a large module from the PowerShell Gallery.
3. ARM-TTK — git clone --depth 1 https://github.com/Azure/arm-ttk.git — network clone every time.

All three steps run serially and re-execute on every container rebuild, because they live in postCreateCommand rather than being baked into the image or cached via onCreateCommand / prebuilds.

Suggested fixes

• Move installs into the image via a Dockerfile.
• Use onCreateCommand instead of postCreateCommand for heavy/static installs.
• Enable Codespaces prebuilds.
• Run installs in parallel (& + wait).
• Cache / pin ARM-TTK and skip clone if already present.
• Consider a devcontainer Feature for ARM-TTK / PSRule.

Acceptance criteria

• Time from "container created" to "ready to use" is significantly reduced (target: <30s steady state).
• All existing tools (Checkov, PSRule.Rules.Azure, ARM-TTK) remain available with equivalent versions/behavior.
• post-create.sh (or its replacement) is idempotent