Determine default cache map from Dockerfile

[bennesp]

Closes #48 (thank you @omus for sharing a very useful code snippet)

This PR allows users to avoid specifying cache-map: the cache-map will be automatically created by inspecting the AST of the Dockerfile. Of course, users can continue specifying cache-map, in situations where users need more control.

Users can specify a Dockerfile of their choice with the dockerfile option.

For each cache mount, the respective generated cache-map entry is the following:

• cache-map source: The id of the cache mount if specified, otherwise the target of the cache mount
• cache-map target: The absolute path of the target (prefixed by the WORKDIR, if any)

Example:

FROM alpine:latest AS builder

# Target absolute path, no id
RUN --mount=type=cache,target=/tmp/cache \
    echo "Hello, World!" > /tmp/cache/hello.txt

# Target relative path with workdir, no id
WORKDIR /app
RUN --mount=type=cache,target=cache1 \
    echo "Hello, World!" > cache/hello.txt

# Multi-stage build
FROM alpine:latest

# Target absolute path with id
RUN --mount=type=cache,id=cache2,target=/tmp/cache \
    echo "Hello, World!" > /tmp/cache/hello.txt

# Target relative path with workdir and id
WORKDIR /app2
RUN --mount=type=cache,id=cache3,target=cache \
    echo "Hello, World!" > cache/hello.txt

This dockerfile will generate the following cache-map:

{
  "/tmp/cache": {
      "id": "/tmp/cache",
      "target": "/var/cache-target"
  },
  "cache1": {
      "id": "cache1",
      "target": "/var/cache-target"
  },
  "cache2": {
      "id": "cache2",
      "target": "/var/cache-target"
  },
  "cache3": {
      "id": "cache3",
      "target": "/var/cache-target"
  }
}

[omus]

Thanks!

[bennesp]

@AkihiroSuda do you think you could kindly take a look at this PR? 🙏

[AkihiroSuda]

Thanks

[AkihiroSuda]

Next time please consider squashing commits

[bennesp]

Thank you for the quick response, I will squash next time!
Best