Support docker build --build-context parameter

[jonnyry]

What does this change

Add new --build-context parameter to porter, and pass through the value to docker build.

https://github.com/docker/buildx/blob/master/docs/reference/buildx_build.md#build-context

What issue does it fix

Closes #3271

Checklist

• Did you write tests?
• Did you write documentation?

[schristoff]

Some small comment but otherwise great work, thank you

[schristoff]

Just checking, are these all valid from Dockers side? I figure if you pass in "C=" and Docker tries to run it it's gonna error, no?

[jonnyry]

Ah yes good point, "C=" is not.

I've taken another look and found docker buildkit has its own ParseContextNames function with error handling (similar to the --ssh param parsing), so I've replaced my hand rolled parseBuildContexts with it, so we get dockers own error handling on the parameter.

This yields the following outputs from porter:

$ ./porter build --build-context
flag needs an argument: --build-context

$ ./porter build --build-context context1
Copying porter runtime ===> 
Copying mixins ===> 
Copying mixin exec ===> 
Copying mixin terraform ===> 
Copying mixin az ===> 
Building bundle image
error parsing the --build-context flags: invalid context value: context1, expected key=value
unable to build bundle image: error parsing the --build-context flags: invalid context value: context1, expected key=value
unable to build bundle image: error parsing the --build-context flags: invalid context value: context1, expected key=value


$ ./porter build --build-context context1=
Copying porter runtime ===> 
Copying mixins ===> 
Copying mixin exec ===> 
Copying mixin terraform ===> 
Copying mixin az ===> 
Building bundle image
[+] Building 0.0s (0/0)                                                                                                                                                                                                                     
error building docker image: failed to get build context context1: stat : no such file or directory
unable to build bundle image: error building docker image: failed to get build context context1: stat : no such file or directory
unable to build bundle image: error building docker image: failed to get build context context1: stat : no such file or directory


./porter build --build-context context1=..
(command successful)

[schristoff]

Good catch, thanks for that update :)