Nit: Fix indentation

[ville-j-koskela]

Fix the indentation on line 12 in docker-compose.yml.

$ yamllint docker-compose.yml 
  1:1       warning  missing document start "---"  (document-start)
  12:8      error    wrong indentation: expected 6 but found 7  (indentation)
  99:81     error    line too long (100 > 80 characters)  (line-length)
  127:81    error    line too long (102 > 80 characters)  (line-length)

[ville-j-koskela]

Closing. Looks like main is the default branch in this repo. Let me cut a new PR.