Reverted not including build args in cache key

[dzoeteman]

#639 removed adding the build args to the cache key. This caused build args to not be considered in at least certain scenarios for caching. An example:

ARG testarg='false'
RUN if [ "$testarg" = 'false' ]; then \
        echo "test1" > /testfile; \
    else \
        echo "test2" > /testfile; \
    fi

Running this build without build arg first, would result in test1 in the /testfile file. However, on subsequent builds with caching, even when including --build-arg "testarg=true", would still result to test1 in the /testfile file.

I've decided to simply revert the change. This seems to work correctly, and cached layers are still used if the build args are not different.
While a better solution might need to be researched long-term, I think this is not a bad solution to fix the current regression.

Fixes #690
Fixes #728

[dzoeteman]

@priyawadhwa You mentioned in #639 (comment) to cc you for this fix, it would be great if you (or someone else) could take a look at this. Thanks :)

[priyawadhwa]

cool, thanks for fixing this!