Fix the CI failures

[julienrf]

• Use the in-memory mode for DynamoDB local
• Make sure the AWS client is fully configured
• Fix filesystem permission issues
• Cache Docker images

[julienrf]

This PR is now ready to be merged. It could even be merged to master, by the way.

[tarzanek]

thank you sir! merging

[tarzanek]

will merge this branch to master too