Build error because missing amazon-linux-extras

[dzygann]

Hi,

I followed your description in the README.md and run into an error during enclavectl build --image hello. It seems that in the latest amazonlinux version is no amazon-linux-extras. I could fix the issue by adding the version in the Dockerfile.builder.

Here is the output of the command.

$~/workspaces/aws/aws-nitro-enclaves-with-k8s$ ./enclavectl build --image hello
[+] Building 0.8s (6/8)
 => [internal] load build definition from Dockerfile.builder                                                                                                                                                   0.0s
 => => transferring dockerfile: 360B                                                                                                                                                                           0.0s
 => [internal] load .dockerignore                                                                                                                                                                              0.0s
 => => transferring context: 2B                                                                                                                                                                                0.0s
 => [internal] load metadata for docker.io/library/amazonlinux:latest                                                                                                                                          0.4s
 => CACHED [1/4] FROM docker.io/library/amazonlinux@sha256:85c8f169ab712d0c5f3fc6616b851b9b9ad0f895daf465fa1bcc95185697bf31                                                                                    0.0s
 => [internal] load build context                                                                                                                                                                              0.0s
 => => transferring context: 63B                                                                                                                                                                               0.0s
 => ERROR [2/4] RUN amazon-linux-extras install aws-nitro-enclaves-cli &&     yum install wget git aws-nitro-enclaves-cli-devel -y                                                                             0.4s
------
 > [2/4] RUN amazon-linux-extras install aws-nitro-enclaves-cli &&     yum install wget git aws-nitro-enclaves-cli-devel -y:
#0 0.338 /bin/sh: line 1: amazon-linux-extras: command not found
------
Dockerfile.builder:6
--------------------
   5 |
   6 | >>> RUN amazon-linux-extras install aws-nitro-enclaves-cli && \
   7 | >>>     yum install wget git aws-nitro-enclaves-cli-devel -y
   8 |
--------------------
error: failed to solve: process "/bin/sh -c amazon-linux-extras install aws-nitro-enclaves-cli &&     yum install wget git aws-nitro-enclaves-cli-devel -y" did not complete successfully: exit code: 127
Unable to find image 'ne-example-builder:latest' locally
docker: Error response from daemon: pull access denied for ne-example-builder, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.
See 'docker run --help'.
[enclavectl] Enclave apps build failed with error. (Code: 125)
[enclavectl] Cannot build enclave applications for hello!

I'm going to create a PR with the fix. Feel free to merge it or to decline it 😄