Dockerfile feature parity

[denzp]

There are plenty of missing features that might block migration from Dockerfile-approach.
Some of the features are trivial to implement, others might require extra work and refactoring.

Builder:

• Custom setup commands (RUN directives before cargo build) Implement custom setup commands (Dockerfile's RUN) #15
• Static assets from the build context (COPY directives) Copy files from context into builder or output images (Dockerfile's COPY) #18
• .dockerignore .dockerignore #16

Output:

• Labels Advanced image metadata #17
• Exposed ports Advanced image metadata #17
• Volumes Advanced image metadata #17
• Stop signal Advanced image metadata #17
• Static assets from the build context (COPY directives) Copy files from context into builder or output images (Dockerfile's COPY) #18
• Custom pre-setup commands (RUN directives before copying build outputs) Implement custom setup commands (Dockerfile's RUN) #15
• Custom post-setup commands (RUN directives after copying build outputs) Implement custom setup commands (Dockerfile's RUN) #15