Allow simple string images

[gwdp]

What did you implement:

Amend fixes on #825

Issue introduced with image command detection implemented on recent versions, preventing simple remote images to be used when in usage with this plugin.
Serverless documentation also indicates its support. (Please, scroll to the very bottom, the last example) https://www.serverless.com/blog/container-support-for-lambda

How did you implement it:

Prevent inclusion of the 'image' into webpack entries, and also detect the image string before checking for the image name.

How can we verify it:

service: report-support
provider:
  name: aws
  region: ca-central-1
  runtime: nodejs12.x
  stage: ${opt:stage, 'dev'}
  versionFunctions: false
plugins:
  - serverless-webpack
functions:
  server:
    image: public.ecr.aws/lambda/nodejs:latest
custom:
  webpack:
    webpackConfig: api/webpack.config.js
    packager: yarn
    keepOutputDirectory: true

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[vicary]

@gwdp please rebase after #967 to fix the failing test.

[gwdp]

@vicary Could you check now please?

[vicary]

lgtm