Build apps with BuildKit so build stages stop piling up (closes #220)

The classic builder left an untagged image for every step of every
stage of a multi-stage build. BuildKit keeps stages in Docker's build
cache, which Docker keeps under a size limit on its own. BuildKit
reports progress as encoded trace messages, so these are decoded with
BuildKit's own library and written to the deployment log as plain text.
The README notes that engines older than 28.2 need build cache cleanup
turned on in daemon.json.

Model: opus-5-5

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/gorilla/sessions v1.4.0
 	github.com/joho/godotenv v1.5.1
 	github.com/mattn/go-sqlite3 v1.14.32
+	github.com/moby/buildkit v0.16.0
 	github.com/oklog/ulid/v2 v2.1.1
 	github.com/prometheus/client_golang v1.23.2
 	github.com/spf13/viper v1.21.0
@@ -24,10 +25,19 @@ require (
 
 require (
 	github.com/AdaLogics/go-fuzz-headers v0.0.0-20240806141605-e8a1dd7889d6 // indirect
+	github.com/Azure/go-ansiterm v0.0.0-20250102033503-faa5f7b0171c // indirect
 	github.com/Microsoft/go-winio v0.6.2 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/containerd/console v1.0.4 // indirect
+	github.com/containerd/containerd v1.7.21 // indirect
+	github.com/containerd/containerd/api v1.7.19 // indirect
+	github.com/containerd/continuity v0.4.3 // indirect
+	github.com/containerd/errdefs v0.1.0 // indirect
 	github.com/containerd/log v0.1.0 // indirect
+	github.com/containerd/platforms v0.2.1 // indirect
+	github.com/containerd/ttrpc v1.2.5 // indirect
+	github.com/containerd/typeurl/v2 v2.2.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/distribution/reference v0.6.0 // indirect
 	github.com/docker/go-units v0.5.0 // indirect
@@ -36,12 +46,23 @@ require (
 	github.com/go-logr/logr v1.4.3 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
+	github.com/gofrs/flock v0.12.1 // indirect
+	github.com/gogo/googleapis v1.4.1 // indirect
 	github.com/gogo/protobuf v1.3.2 // indirect
+	github.com/golang/protobuf v1.5.4 // indirect
+	github.com/google/go-cmp v0.7.0 // indirect
+	github.com/google/shlex v0.0.0-20191202100458-e7afc7fbc510 // indirect
 	github.com/gorilla/securecookie v1.1.2 // indirect
+	github.com/grpc-ecosystem/grpc-gateway/v2 v2.27.3 // indirect
+	github.com/hashicorp/errwrap v1.1.0 // indirect
+	github.com/hashicorp/go-multierror v1.1.1 // indirect
+	github.com/in-toto/in-toto-golang v0.5.0 // indirect
 	github.com/klauspost/compress v1.18.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect
+	github.com/moby/locker v1.0.1 // indirect
 	github.com/moby/patternmatcher v0.6.0 // indirect
 	github.com/moby/sys/sequential v0.6.0 // indirect
+	github.com/moby/sys/signal v0.7.1 // indirect
 	github.com/moby/sys/user v0.4.0 // indirect
 	github.com/moby/sys/userns v0.1.0 // indirect
 	github.com/moby/term v0.5.2 // indirect
@@ -56,25 +77,42 @@ require (
 	github.com/prometheus/common v0.66.1 // indirect
 	github.com/prometheus/procfs v0.16.1 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
+	github.com/secure-systems-lab/go-securesystemslib v0.4.0 // indirect
+	github.com/shibumi/go-pathspec v1.3.0 // indirect
 	github.com/sirupsen/logrus v1.9.3 // indirect
 	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
 	github.com/spf13/afero v1.15.0 // indirect
 	github.com/spf13/cast v1.10.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
+	github.com/tonistiigi/fsutil v0.0.0-20240424095704-91a3fc46842c // indirect
+	github.com/tonistiigi/go-csvvalue v0.0.0-20240710180619-ddb21b71c0b4 // indirect
+	github.com/tonistiigi/units v0.0.0-20180711220420-6950e57a87ea // indirect
+	github.com/tonistiigi/vt100 v0.0.0-20240514184818-90bafcd6abab // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect
+	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.46.1 // indirect
+	go.opentelemetry.io/contrib/instrumentation/net/http/httptrace/otelhttptrace v0.46.1 // indirect
 	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.64.0 // indirect
 	go.opentelemetry.io/otel v1.39.0 // indirect
+	go.opentelemetry.io/otel/exporters/otlp/otlptrace v1.39.0 // indirect
 	go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp v1.39.0 // indirect
 	go.opentelemetry.io/otel/metric v1.39.0 // indirect
+	go.opentelemetry.io/otel/sdk v1.39.0 // indirect
 	go.opentelemetry.io/otel/trace v1.39.0 // indirect
+	go.opentelemetry.io/proto/otlp v1.9.0 // indirect
 	go.uber.org/dig v1.19.0 // indirect
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
+	golang.org/x/net v0.47.0 // indirect
+	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.39.0 // indirect
 	golang.org/x/text v0.32.0 // indirect
+	google.golang.org/genproto v0.0.0-20240123012728-ef4313101c80 // indirect
+	google.golang.org/genproto/googleapis/api v0.0.0-20251202230838-ff82c1b0f217 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
+	google.golang.org/grpc v1.77.0 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 	gotest.tools/v3 v3.5.2 // indirect