docker (compose): test environment variable + network alias: `SAT_TEST_ENV_E2E` is set to `1` to indicate that e2e environment requirement are available (notably specific accounts and hosts created). The e2e tests needing it won't be launched if this variable is not set. A network alias is set for the backend at `sat.test` (to have a tld available, needed for some network libraries like OpenSSL).

[DEFAULT]
media_dir = /home/sat/sat_media

[jp]
# FIXME: default "auto" value cause trouble
background = dark