nit: fix indentation in docker-compose

[ville-j-koskela]

Fix the indentation in the docker-compose.yml file.

$ yamllint docker-compose.yml 
docker-compose.yml
  1:1       warning  missing document start "---"  (document-start)
  12:81     error    line too long (81 > 80 characters)  (line-length)
  32:8      error    wrong indentation: expected 6 but found 7  (indentation)
  43:81     error    line too long (81 > 80 characters)  (line-length)
  61:8      error    wrong indentation: expected 6 but found 7  (indentation)
  73:8      error    wrong indentation: expected 6 but found 7  (indentation)
  148:81    error    line too long (100 > 80 characters)  (line-length)
  174:81    error    line too long (102 > 80 characters)  (line-length)