Update our Docker layer caching docs to avoid using SEMAPHORE_GIT_BRANCH

[csidyel]

Coming from https://github.com/renderedtext/issues/issues/3174

We'd need to update our docs to avoid using this env var. What could be added instead? Any ideas?

[bogyo210]

@csidyel @CommanderK5
Please share your thoughts on:
echo -n $SEMAPHORE_GIT_BRANCH | md5sum | awk '{print $1}'
Maybe we could introduce that env var :)

[csidyel]

Even though it does seem useful, I'm not sure how it would look in our docs.
Looking at this value maybe something already present in the platform like SEMAPHORE_GIT_SHA could help?