Specify context for docker run command

[elazarl]

When building with modern docker, the docker build would run with an empty context, if no directory is specified.

Hence the docker build would fail finding repository's files:

 docker build -t seastar-dev docker/dev
Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM fedora:29
29: Pulling from library/fedora
01eb078129a0: Pull complete
Digest: sha256:2c20e5bb324735427f8a659e36f4fe14d6955c74c7baa25067418dddbb71d67a
Status: Downloaded newer image for fedora:29
 ---> d09302f77cfc
Step 2/3 : COPY install-dependencies.sh .
COPY failed: stat /var/lib/docker/tmp/docker-builder314166541/install-dependencies.sh: no such file or directory

The new command would specify the docker build context as the seastar git repository, and specify where the Dockerfile in the repository is.