Handle container lifecycle events through eventbridge

[epicfaace]

Fixes #257.

Adds infrastructure for handling events (task start, task stop) through EventBridge. When an event is called, it calls the updateScanTaskStatus lambda function, which updates the scantask's status accordingly.

This approach is better than the earlier approach because we can be sure we catch all errors (previously, sometimes if the entire process crashed, the db wouldn't update that the task had stopped and it would be marked as "started" forever).

Local development

For local development, there is a handler that listens to Docker events (container start / stop) and then invokes updateScanTaskStatus with the right payload. This handler runs on the backend container during local development.

Logs

One change is that the output column no longer contains the error directly, as that is not returned from the EventBridge stopped event. Logs can now be accessed by directly going to a link on CloudWatch on the AWS console. Perhaps in the future we can show logs directly in Crossfeed by pulling data directly from CloudWatch.

Testing

I've deployed this on https://staging.crossfeed.cyber.dhs.gov/ and it seems to work well.

[lgtm-com]

This pull request introduces 1 alert when merging 3699aae into 5bb9d69 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 1 alert when merging dc0136c into 5bb9d69 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 3 alerts and fixes 1 when merging ae39fa7 into 5bb9d69 - view on LGTM.com

new alerts:

• 2 for Unused variable, import, function or class
• 1 for Superfluous trailing arguments

fixed alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 3 alerts and fixes 1 when merging 512c83d into 5bb9d69 - view on LGTM.com

new alerts:

• 2 for Unused variable, import, function or class
• 1 for Superfluous trailing arguments

fixed alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 79719b1 into 5bb9d69 - view on LGTM.com

new alerts:

• 1 for Superfluous trailing arguments

fixed alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging ee55c2a into 5bb9d69 - view on LGTM.com

new alerts:

• 1 for Superfluous trailing arguments

fixed alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 843867e into 5bb9d69 - view on LGTM.com

new alerts:

• 1 for Superfluous trailing arguments

fixed alerts:

• 1 for Unused variable, import, function or class

[epicfaace]

@cablej can you take a look when you get a chance? I tested this on the old staging environment and everything seemed to work.

[cablej]

This looks good, we can consider fetching logs inline in the future