Description of your changes

This PR modifies the Gitpod configuration to pin a specific base image for the workspace and configure dependencies in it.

Prior to this PR Gitpod was starting a workspace using the latest image available, which in some cases like now it's not desirable. Following the same practices that we use in other areas like dependencies and GitHub Actions, I have changed the configuration so that we are pinning a specific version of a specific Docker image identified by its sha number. I got this idea from the Python repository which does the same.

The new setup specifies a Dockerfile which uses the base workspace image and builds on top of it. I decided not to use the node image provided by Gitpod because I wanted to customize the setup of the Node.js version manager using fnm instead of nvm. In doing so I have also moved the SAM CLI install to the Dockerfile.

Related issues, RFCs

Issue number: #1781

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my change is effective and works
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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