SHELL switch not supported while building docker image.

[erwan-koffi]

Describe the bug
SHELL command in Dockerfile not supported.

To Reproduce
Steps to reproduce the behavior:
Use the following docker image:

FROM centos:8

SHELL ["/bin/bash", "-c"]

Run the makisu_build -t base .

Expected behavior
The SHELL command should be parsed and used for the build of the image.

Screenshots
If applicable, add screenshots to help explain your problem.

Environments
K8s and Laptop

Additional context
Error message:
{"level":"error","ts":1611580483.2189858,"msg":"failed to create build plan: failed to get dockerfile: failed to parse dockerfile: failed to create new directive (line 2): failed to parse 'SHELL' directive with args '[\"/bin/bash\", \"-c\"]': Unsupported directive type"}