build: make the compile stages cacheable across machines

.dockerignore

@@ -1,3 +1,6 @@
 bin
 .certs
 .tmp
+# See .earthlyignore for why .git is excluded. Kept in sync here so plain
+# docker builds get the same cacheable context.
+.git

.earthlyignore

@@ -0,0 +1,18 @@
+# .git is deliberately excluded from the build context.
+#
+# Three Dockerfile stages mount the context (buildkit-version, buildctl,
+# buildkitd). A git checkout is not byte-reproducible -- .git/index carries
+# mtimes, logs/HEAD carries timestamps, packed-refs ordering varies -- so
+# including it made the context digest differ per machine, and those three
+# stages missed the layer cache on every machine that had not already built
+# them. runc, which mounts from the runc-src stage instead of the context, was
+# the only compile stage that reused a shared cache.
+#
+# Nothing in the build needs .git any more: buildkit-version now takes REVISION
+# as a build arg instead of shelling out to git.
+#
+# NOTE: the Earthfile here is VERSION 0.6, and earthly only falls back to
+# .dockerignore from 0.8 (the use-docker-ignore feature). .earthlyignore is
+# honoured at every version, so it is what actually takes effect for
+# `FROM DOCKERFILE`; .dockerignore is kept in sync for plain docker builds.
+.git

Dockerfile

@@ -85,8 +85,23 @@ ENV GOFLAGS=-mod=vendor
 FROM buildkit-base AS buildkit-version
 # TODO: PKG should be inferred from go modules
 ARG RELEASE_VERSION=v0.0.0+earthlyunknown
-RUN --mount=target=. \
-  PKG=github.com/moby/buildkit EARTHLY_PKG=github.com/EarthBuild/buildkit VERSION=$(git describe --match 'v[0-9]*' --dirty='.m' --always --tags) REVISION=$(git rev-parse HEAD)$(if ! git diff --no-ext-diff --quiet --exit-code; then echo .m; fi); \
+# REVISION is supplied by the caller rather than derived from git here.
+#
+# Deriving it needed `--mount=target=.`, which put the whole build context --
+# including .git -- into this stage's cache key. A git checkout is not
+# byte-reproducible, so the context digest differed per machine and this stage
+# missed the cache everywhere it had not already been built. buildctl and
+# buildkitd mount /tmp/.ldflags from here, so they inherited the miss; runc,
+# which mounts from runc-src instead of the context, was the only compile stage
+# that reused a shared cache. .git is now excluded from the context entirely
+# (see .earthlyignore), which is what makes the two expensive compile stages
+# cacheable across machines.
+#
+# The old `VERSION=$(git describe ...)` was already dead: the ldflags line has
+# used ${RELEASE_VERSION} since "build: bake version.Package as
+# github.com/EarthBuild/buildkit" (#18), so the computed value was discarded.
+ARG REVISION=unknown
+RUN PKG=github.com/moby/buildkit EARTHLY_PKG=github.com/EarthBuild/buildkit; \
   echo "-X ${PKG}/version.Version=${RELEASE_VERSION} -X ${PKG}/version.Revision=${REVISION} -X ${PKG}/version.Package=${EARTHLY_PKG}" | tee /tmp/.ldflags; \
   echo -n "${RELEASE_VERSION}" | tee /tmp/.version;
 

Earthfile

@@ -7,7 +7,12 @@ WORKDIR /buildkit
 # build buildkit image
 build:
     ARG RELEASE_VERSION=v0.0.0+earthlyunknown
-    FROM DOCKERFILE --build-arg RELEASE_VERSION=$RELEASE_VERSION --target buildkit-linux .
+    # REVISION is baked into `buildkitd --version` output. It used to be read
+    # from .git inside the buildkit-version stage; see the comment there for why
+    # that is now passed in instead. Callers that want a real revision should
+    # pass the pinned buildkit git sha they are building.
+    ARG REVISION=unknown
+    FROM DOCKERFILE --build-arg RELEASE_VERSION=$RELEASE_VERSION --build-arg REVISION=$REVISION --target buildkit-linux .
 
 code:
     COPY . .