Pin Dockerfile to -stretch variant
The Golang base images switch to buster, which causes some breakage
in networking and packages that are no longer available; (`btrfs-tools`
is now an empty package, and `libprotobuf-c0-dev` is gone).
Some of out tests also start faiilng on stretch, and will have to be
investigated further;
```
15:13:06 --- FAIL: TestRenameAnonymousContainer (3.37s)
15:13:06 rename_test.go:168: assertion failed: 0 (int) != 1 (inspect.State.ExitCode int): container a7fe866d588d65f353f42ffc5ea5288e52700384e1d90850e9c3d4dce8657666 exited with the wrong exitcode:
15:13:38 --- FAIL: TestHostnameDnsResolution (2.23s)
15:13:38 run_linux_test.go:128: assertion failed:
15:13:38 --- ←
15:13:38 +++ →
15:13:38 @@ -1 +1,2 @@
15:13:38 +ping: bad address 'foobar'
15:13:38
15:13:38
15:13:38 run_linux_test.go:129: assertion failed: 0 (int) != 1 (res.ExitCode int)
```
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit ed672bb523cb255d0b2b79837d9c45a7c3255000)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Sebastiaan van Stijn