Silly experiments with RUN --mount

relates to moby/moby#32507, moby/buildkit#442

Doing some silly experimenting with RUN --mount:

# syntax = docker/dockerfile:experimental

FROM alpine AS stage1
RUN mkdir -p /stage1-files
RUN echo "testing stage 1" > /stage1-files/s1-file

FROM alpine AS stage2
RUN mkdir -p /stage2-files
RUN echo "testing stage 2" > /stage2-files/s2-file

# The "utility" stage mounts "stage1" (readonly), and "stage2" (readwrite),
# processes files from "stage1", and writes the result to "stage2".
#
# The idea here is to have "utility" build-stages that have tools installed
# to manipulate other stages, without those tools ending up in the stage (layer)
# itself
FROM alpine AS utility
RUN --mount=from=stage1,dst=/stage1 --mount=from=stage2,dst=/stage2,readwrite cp -r /stage1/stage1-files /stage2/

# this "utility" stage processes files from the "stage1" stage, mounting it
# read-write to make modifications
FROM alpine AS utility2
RUN --mount=from=stage1,dst=/stage1,readwrite touch /stage1/stage1-files/s1-file2

# this of course works
FROM alpine AS utility3
RUN mkdir /utility3-files
RUN --mount=from=stage1,dst=/stage1 cp -r /stage1/stage1-files /utility3-files/

# this doesn't work: this still gives the original, unmodified layer from stage1
FROM stage1 AS attempt1
RUN apk add --no-cache tree
CMD tree /stage1-files

# this doesn't work for stage1 and 2: --mount still gives the original,
# unmodified layers from those stages
FROM alpine AS attempt2
RUN apk add --no-cache tree
RUN mkdir -p /results/stage1-result /results/stage2-result /results/utility3-result
RUN --mount=from=stage1,dst=/stage1 cp -r /stage1/stage1-files /results/stage1-result
RUN --mount=from=stage2,dst=/stage2 cp -r /stage2/stage2-files /results/stage2-result
RUN --mount=from=utility3,dst=/utility3 cp -r /utility3/utility3-files /results/utility3-result
CMD tree /results

Building works (no errors):

docker build --no-cache -t bla .

[+] Building 4.7s (18/18) FINISHED                                                                                                                                            
 => local://dockerfile (Dockerfile)                                                                                                                                      0.0s
 => => transferring dockerfile: 1.88kB                                                                                                                                   0.0s
 => local://context (.dockerignore)                                                                                                                                      0.0s
 => => transferring context: 02B                                                                                                                                         0.0s
 => docker-image://docker.io/tonistiigi/dockerfile:runmount20180618@sha256:576332cea88216b4bf20c56046fabb150c675be4a504440da11970bea501281b                              0.0s
 => => resolve docker.io/tonistiigi/dockerfile:runmount20180618@sha256:576332cea88216b4bf20c56046fabb150c675be4a504440da11970bea501281b                                  0.0s
 => => sha256:576332cea88216b4bf20c56046fabb150c675be4a504440da11970bea501281b 528B / 528B                                                                               0.0s
 => => sha256:d0fbaded5db6066249af00e1c83c06c976dc9ba74bfca3d5efee1c7856253aa3 1.58kB / 1.58kB                                                                           0.0s
 => local://dockerfile (Dockerfile)                                                                                                                                      0.0s
 => local://context (.dockerignore)                                                                                                                                      0.0s
 => CACHED docker-image://docker.io/library/alpine:latest                                                                                                                0.0s
 => /bin/sh -c mkdir -p /stage2-files                                                                                                                                    0.4s
 => /bin/sh -c mkdir -p /stage1-files                                                                                                                                    0.5s
 => /bin/sh -c mkdir /utility3-files                                                                                                                                     0.6s
 => /bin/sh -c apk add --no-cache tree                                                                                                                                   1.0s
 => /bin/sh -c echo "testing stage 2" > /stage2-files/s2-file                                                                                                            0.5s
 => /bin/sh -c echo "testing stage 1" > /stage1-files/s1-file                                                                                                            0.4s
 => /bin/sh -c mkdir -p /results/stage1-result /results/stage2-result /results/utility3-result                                                                           0.5s
 => /bin/sh -c cp -r /stage1/stage1-files /utility3-files/                                                                                                               0.5s
 => /bin/sh -c cp -r /stage1/stage1-files /results/stage1-result                                                                                                         0.4s
 => /bin/sh -c cp -r /stage2/stage2-files /results/stage2-result                                                                                                         0.4s
 => /bin/sh -c cp -r /utility3/utility3-files /results/utility3-result                                                                                                   0.5s
 => exporting to image                                                                                                                                                   0.0s
 => => exporting layers                                                                                                                                                  0.0s
 => => writing image sha256:3ef6a42407019d37b5d13c9be1685e3ce2fabdf4c4f3d3fa294fc64e7836ded7                                                                             0.0s
 => => naming to docker.io/library/bla                                                                                                                                   0.0s

Running the resulting image produces:

docker run --rm bla

/results
├── stage1-result
│   └── stage1-files
│       └── s1-file
├── stage2-result
│   └── stage2-files
│       └── s2-file
└── utility3-result
    └── utility3-files
        └── stage1-files
            └── s1-file

7 directories, 3 files

It's obvious from the above that I don't have a clue what/how the readwrite option is used (in combination with from=<stage|image>)