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).