Pull Endpoints image if it doesn't exist

[efekarakus]

Description of changes:
As a user that's running ecs-cli local up and doesn't have the amazon/amazon-ecs-local-container-endpoints downloaded locally.
I want ecs-cli local up to pull the image for me.
So that I don't have to pull the image myself.

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Enter [N/A] in the box, if an item is not applicable to your change.

Testing

• Unit tests passed
• Integration tests passed
• Unit tests added for new functionality
• Listed manual checks and their outputs in the comments (example)
• [N/A] Link to issue or PR for the integration tests: The integ tests need to pull the image to pass.

Documentation

• Contacted our doc writer
• Updated our README

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[efekarakus]

Manual Tests

1. Image is not available locally

$ docker images -f reference=amazon/amazon-ecs-local-container-endpoints                  
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
$ ecs-cli local up
INFO[0000] Creating network: ecs-local-network...       
INFO[0015] Created network ecs-local-network with ID b42fe58c8a43cdd61c5c25baf0f83574803f0746df155dd84edfb3c6693e3024 
INFO[0015] Pulling image amazon/amazon-ecs-local-container-endpoints 
INFO[0018] Pulled image amazon/amazon-ecs-local-container-endpoints 
INFO[0018] Created the amazon-ecs-local-container-endpoints container with ID fecd0d8bb61397c5882dbc6905053b6181bcb6e3654997a0e046f8198c46ffe2 
INFO[0019] Started container with ID fecd0d8bb61397c5882dbc6905053b6181bcb6e3654997a0e046f8198c46ffe2

2. Image exists and we do "local up"

$ docker images -f reference=amazon/amazon-ecs-local-container-endpoints               
REPOSITORY                                    TAG                 IMAGE ID            CREATED             SIZE
amazon/amazon-ecs-local-container-endpoints   latest              ccb85faf9315        7 weeks ago         17.4MB
$ ecs-cli local up
INFO[0000] The network ecs-local-network already exists 
INFO[0000] The amazon-ecs-local-container-endpoints container already exists with ID fecd0d8bb61397c5882dbc6905053b6181bcb6e3654997a0e046f8198c46ffe2 
INFO[0000] Started container with ID fecd0d8bb61397c5882dbc6905053b6181bcb6e3654997a0e046f8198c46ffe2 

3. Image download fails due to a network failure

I turned off my wifi while downloading the image here is the experience:

$ ecs-cli local up                        
INFO[0000] The network ecs-local-network already exists 
INFO[0000] Pulling image amazon/amazon-ecs-local-container-endpoints 
FATA[0120] Failed to download the image amazon/amazon-ecs-local-container-endpoints due to context deadline exceeded 

[PettitWesley]

How does this work?
https://golang.org/pkg/io/ioutil/#ReadAll seems to return two values, both of which are ignored here...

[efekarakus]

After our discussion offline, I tested with turning off the wifi during img download (see 3. under Manual Tests). Added an error check here, thanks for pointing it out :)

[PettitWesley]

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