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fix: Expand ARG and ENV variables when parsing Dockerfile #131

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fix: Expand ARG and ENV variables when parsing Dockerfile #131

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31 changes: 27 additions & 4 deletions pkg/dockerfile/parse.go
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@ package dockerfile
import (
"bufio"
"io"
"os"
"strconv"
"strings"
"unicode"
Expand All @@ -27,6 +28,15 @@ func ParseReader(dockerfile io.Reader) (Metadata, error) {
// (nil slices work fine)
}

// Store the arg and env instructions for later variable expansion
env := make(map[string]string)
resolve := func(k string) string {
if v, found := env[k]; found {
return v
}
return k
}

scanner := bufio.NewScanner(dockerfile)
for scanner.Scan() {
line := strings.TrimSpace(scanner.Text())
Expand Down Expand Up @@ -77,11 +87,21 @@ func ParseReader(dockerfile io.Reader) (Metadata, error) {

instruction := strings.ToUpper(fields[0])

// TODO balk at ARG / $ in from values

switch instruction {
case "ARG", "ENV":
switch {
// Handle new style "ENV FOO=BAR" assignment style
case len(fields) == 2 && strings.Contains(fields[1], "="):
if k, v, found := strings.Cut(fields[1], "="); found {
env[k] = v
}
// Handle old style "ENV FOO BAR" assignment style
case len(fields) == 3 && !strings.Contains(fields[1], "="):
env[fields[1]] = fields[3]
}
case "FROM":
from := fields[1]
// Expand variables
from := os.Expand(fields[1], resolve)

if stageFrom, ok := meta.StageNameFroms[from]; ok {
// if this is a valid stage name, we should resolve it back to the original FROM value of that previous stage (we don't care about inter-stage dependencies for the purposes of either tag dependency calculation or tag building -- just how many there are and what external things they require)
Expand All @@ -101,7 +121,10 @@ func ParseReader(dockerfile io.Reader) (Metadata, error) {
}

case "COPY":
for _, arg := range fields[1:] {
for _, f := range fields[1:] {
// Expand variables
arg := os.Expand(f, resolve)

if !strings.HasPrefix(arg, "--") {
// doesn't appear to be a "flag"; time to bail!
break
Expand Down