Make output window configurable: BUILDKIT_TERMHEIGHT

[holzman]

The output window was previously hard-coded to a height of 6; this patch makes it configurable at run-time by setting the BUILDKIT_TERMHEIGHT environment variable.

[holzman]

I like the tty output, but I normally have tall windows and found myself annoyed by only having 6 lines of
output.

[tonistiigi]

Iiuc this is not the window or viewport height (that is detected automatically from tty), but how many log lines are visible for active steps in tty mode (if there is enough room). In that case, the more suitable name would be something like BUILDKIT_TTY_LOG_LINES or smth like that?

[tonistiigi]

I like the tty output, but I normally have tall windows and found myself annoyed by only having 6 lines of
output.

Theoretically, this could be somewhat automatically detected by counting the active lines and comparing them with the screen height. Eg. if the screen height is 40 lines, the whole tty output is 10 lines, and there is only one active step then indeed the logs for the active step should not be limited by 6 lines but could take 2-3 times more room. If more lines go active or the screen is resized then it should reduce the limit though. Atm. this logic is more simplified, every active step gets a maximum of 6 lines and if it doesn't fit anymore then the logs for some steps are not shown anymore.

[holzman]

Iiuc this is not the window or viewport height (that is detected automatically from tty), but how many log lines are visible for active steps in tty mode (if there is enough room). In that case, the more suitable name would be something like BUILDKIT_TTY_LOG_LINES or smth like that?

Sure, I'll change it. I based the env var on the variable already in the code, which is confusingly named.