plugin/docker: add no-cache option

[radriaanse]

Allow to pass through the no-cache flag to disable the build cache.

This is useful in cases where you want to make sure you're doing a clean build or to mitigate issues with the build cache that arise from time to time.

[radriaanse]

Verified it's working, default behavior stays the same (cache is used):

» Building nocache...
-> Initializing Docker client...
-> Building image...
Step 1/2 : FROM scratch
 --->
Step 2/2 : COPY . .
 ---> Using cache
 ---> c124aba5ef06

And after setting to true the build log shows no cached layers are used:

» Building nocache...
-> Initializing Docker client...
-> Building image...
Step 1/2 : FROM scratch
 --->
Step 2/2 : COPY . .
 ---> c124aba5ef06
Successfully built c124aba5ef06

Also tested with buildkit, seems to work just fine as well but can't verify using logs since those are broken.

[krantzinator]

Look fine to me, thank you!
A nit that doesn't pertain to this PR specifically is that we should probably update the buildWithDocker func to take in options; it's getting a bit unwieldy.